feat: implement proper merge_bands functionality + comprehensive tests

BREAKING CHANGE: merge_bands parameter now correctly creates separate files per band when False

Features:
- Implement proper separate band file saving (merge_bands=False)
- Add 4 comprehensive pytest tests for merge_bands functionality
- Tests verify: single vs separate files, band counts, file shapes

Tests:
- test_merge_bands_geotiff: verifies merge_bands=True creates 1 file with 3 bands
- test_separate_bands_geotiff: verifies merge_bands=False creates 3 files with 1 band each
- test_merge_bands_numpy: verifies 3D array (3,h,w) for merged
- test_separate_bands_numpy: verifies 3 2D arrays (h,w) for separate
- All 53 tests passing (4 new merge_bands tests added)

Documentation:
- Update SAVE_PARAMETERS.md with clear merge_bands explanation
- Remove Provider Selection section from README (no need to compare providers)
- Add merge_bands_demo.py example demonstrating both modes
- Keep models/ directory name (industry standard for Pydantic models)

Quality:
- Run make clean to remove build artifacts
- All linting passing (ruff, flake8, mypy)
- Fixed line length and type errors
- Professional code structure maintained

Author: Kongstad

README.md

@@ -23,7 +23,6 @@ Python package for downloading satellite imagery from multiple sources with a st
   - [SAR for All-Weather Monitoring](#sar-for-all-weather-monitoring)
   - [Multi-Area Batch Processing](#multi-area-batch-processing)
 - [Advanced Features](#advanced-features)
-  - [Provider Selection](#provider-selection)
   - [Overlapping Tile Handling](#overlapping-tile-handling)
   - [S3 Storage](#s3-storage)
 - [Development](#development)
@@ -251,24 +250,6 @@ for idx, area in areas.iterrows():
 
 ## Advanced Features
 
-### Provider Selection
-
-Different providers have different characteristics:
-
-```python
-# Microsoft Planetary Computer (MPC)
-# - Global coverage, always available
-# - Authentication token handled automatically
-processing_params = ProcessingParams(satellite='S2MPC')
-
-# Element84 (E84)
-# - Better for US/Americas
-# - Different band scaling
-processing_params = ProcessingParams(satellite='S2E84')
-```
-
-See [SATELLITE_SOURCES.md](docs/SATELLITE_SOURCES.md) for provider details.
-
 ### Overlapping Tile Handling
 
 By default, overlapping satellite passes from the same day are merged:

docs/SAVE_PARAMETERS.md

@@ -207,25 +207,31 @@ enable_compression=False  # No compression (faster but larger)
 **Required**: No  
 **Default**: `True`
 
-Merge all bands into single multi-band file vs. separate files per band.
+Control whether to save all bands in a single multi-band file or as separate single-band files.
 
 ```python
-merge_bands=True   # Single file with multiple bands (recommended)
+merge_bands=True   # Single multi-band file (recommended)
 merge_bands=False  # Separate file per band
 ```
 
-**Single file** (merge_bands=True):
+**Single multi-band file** (merge_bands=True):
 ```
-S2MPC_2024-06-15_merged_copenhagen.tif
+S2MPC_2024-06-15_merged_copenhagen.tif  # Contains all bands (RGB, NIR, etc.)
 ```
+- GeoTIFF: Single file with multiple bands (e.g., 3 bands for RGB)
+- NumPy: Single 3D array with shape (bands, height, width)
+- Recommended for most use cases
 
-**Separate files** (merge_bands=False):
+**Separate single-band files** (merge_bands=False):
 ```
 S2MPC_2024-06-15_red_copenhagen.tif
 S2MPC_2024-06-15_green_copenhagen.tif
 S2MPC_2024-06-15_blue_copenhagen.tif
 S2MPC_2024-06-15_nir_copenhagen.tif
 ```
+- GeoTIFF: One file per band, each containing a single band
+- NumPy: One 2D array per band with shape (height, width)
+- Useful when processing bands individually or for specialized workflows
 
 ## Naming
 

examples/merge_bands_demo.py

@@ -0,0 +1,100 @@
+"""
+Demonstration of merge_bands parameter functionality.
+
+This example shows the difference between:
+- merge_bands=True: Saves all bands in a single multi-band file
+- merge_bands=False: Saves each band as a separate single-band file
+"""
+
+from shapely.geometry import box
+
+from sat_data_acquisition import ProcessingParams, SaveParams, SatDataClient
+
+# Define a small area of interest (Copenhagen, Denmark)
+bbox = box(12.5683, 55.6761, 12.5783, 55.6811)
+
+# ===== Example 1: Merged Bands (Default) =====
+print("Example 1: Saving as merged multi-band files")
+print("=" * 60)
+
+save_params_merged = SaveParams(
+    output_path="./data/merged_example",
+    save_to_local=True,
+    save_as_geotiff=True,
+    save_as_numpy=True,
+    merge_bands=True,  # Default - saves single multi-band file
+)
+
+processing_params = ProcessingParams(
+    satellite="S2MPC",
+    search_method="geometry",
+    start_date="2024-06-01",
+    end_date="2024-06-05",
+    max_cloud_cover=20,
+    bands=["red", "green", "blue"],
+)
+
+client = SatDataClient()
+
+print("Downloading and saving merged files...")
+for item_data in client.get_data(
+    geometry=bbox,
+    processing_params=processing_params,
+    save_params=save_params_merged,
+    identifier="copenhagen",
+):
+    print(f"  Saved merged file for {item_data['datetime']}")
+
+print("\nOutput structure (merged):")
+print("  data/merged_example/S2MPC/2024/tiff/")
+print("    └── S2MPC_2024-06-02_merged_copenhagen.tif  (3 bands in 1 file)")
+print("  data/merged_example/S2MPC/2024/npy/")
+print("    └── S2MPC_2024-06-02_merged_copenhagen.npy  (shape: 3, h, w)")
+print()
+
+# ===== Example 2: Separate Band Files =====
+print("\nExample 2: Saving as separate band files")
+print("=" * 60)
+
+save_params_separate = SaveParams(
+    output_path="./data/separate_example",
+    save_to_local=True,
+    save_as_geotiff=True,
+    save_as_numpy=True,
+    merge_bands=False,  # Saves separate file per band
+)
+
+print("Downloading and saving separate band files...")
+for item_data in client.get_data(
+    geometry=bbox,
+    processing_params=processing_params,
+    save_params=save_params_separate,
+    identifier="copenhagen",
+):
+    print(f"  Saved separate band files for {item_data['datetime']}")
+
+print("\nOutput structure (separate):")
+print("  data/separate_example/S2MPC/2024/tiff/")
+print("    ├── S2MPC_2024-06-02_red_copenhagen.tif    (1 band)")
+print("    ├── S2MPC_2024-06-02_green_copenhagen.tif  (1 band)")
+print("    └── S2MPC_2024-06-02_blue_copenhagen.tif   (1 band)")
+print("  data/separate_example/S2MPC/2024/npy/")
+print("    ├── S2MPC_2024-06-02_red_copenhagen.npy    (shape: h, w)")
+print("    ├── S2MPC_2024-06-02_green_copenhagen.npy  (shape: h, w)")
+print("    └── S2MPC_2024-06-02_blue_copenhagen.npy   (shape: h, w)")
+print()
+
+# ===== Recommendations =====
+print("\nRecommendations:")
+print("=" * 60)
+print("• Use merge_bands=True (default) for:")
+print("  - Standard remote sensing workflows")
+print("  - Visualization and analysis in QGIS/ArcGIS")
+print("  - Machine learning with multi-band inputs")
+print("  - More efficient storage and faster loading")
+print()
+print("• Use merge_bands=False when:")
+print("  - Processing bands individually")
+print("  - Only using a subset of bands in downstream tasks")
+print("  - Integrating with systems expecting single-band files")
+print("  - Need flexibility to work with bands separately")