perf(gribjump): fix two performance bottlenecks in the gribjump source

Avoid recomputing per-field data by caching/precomputing:
- Cache reference lat/lon when fetch_coords_from_fdb=True to avoid re-reading
  the reference field's geography per retrieved field.
- Pre-convert index lists to ranges once to avoid repeated calls to ExtractionRequest.from_indices.

Author: andreas-grafberger

src/earthkit/data/sources/gribjump.py

@@ -242,6 +242,12 @@ def from_mars_requests(
             ranges = mask_to_ranges(mask)
             mask = None
 
+        if indices is not None:
+            # We do the same small optimization for indices. Optimally, we
+            # would do similar optimizations in pygribjump and remove this.
+            ranges = [(i, i + 1) for i in indices]
+            indices = None
+
         extraction_requests = [build_extraction_request(req, ranges, mask, indices) for req in mars_requests]
         return cls(extraction_requests)
 
@@ -286,7 +292,11 @@ def __init__(
         # These attributes are set lazily after loading the data.
         self._loaded = False
         self._grid_indices = None
+
+        # Cached reference metadata for coordinates
         self._reference_metadata: Optional[GribMetadata] = None
+        self._latitudes: Optional[np.ndarray] = None
+        self._longitudes: Optional[np.ndarray] = None
 
         super().__init__(fields=None)
 
@@ -334,10 +344,10 @@ def _load(self):
 
     def _load_reference_metadata(self):
         """Loads the reference metadata from the FDB retriever if available."""
-        if self._fdb_retriever is None:
-            return None
         if self._reference_metadata is not None:
             return self._reference_metadata
+        if self._fdb_retriever is None:
+            return None
 
         fields = self._fdb_retriever.get(self._requests[0].request)
         metadatas = fields.metadata()
@@ -355,9 +365,13 @@ def _enrich_metadata_with_coordinates(self, indices: np.ndarray, metadata: UserM
         if (reference_metadata := self._load_reference_metadata()) is None:
             return metadata
 
-        reference_geography = reference_metadata.geography
-        grid_latitudes = reference_geography.latitudes()[indices]
-        grid_longitudes = reference_geography.longitudes()[indices]
+        if self._latitudes is None or self._longitudes is None:
+            self._latitudes = reference_metadata.geography.latitudes()
+            self._longitudes = reference_metadata.geography.longitudes()
+
+        grid_latitudes = self._latitudes[indices]
+        grid_longitudes = self._longitudes[indices]
+
         metadata = metadata.override(
             {
                 "latitudes": grid_latitudes,

tests/sources/test_gribjump.py

@@ -192,7 +192,6 @@ def ds_expected_with_coords():
             "levelist": "1000",
             "levtype": "pl",
             "stream": "oper",
-            "param": "129",
             "time": "1200",
             "type": "fc",
             "Conventions": "CF-1.8",