feat: Add PPM (Prediction by Partial Matching) algorithm implementation

- Implemented the PPM algorithm for data compression and decompression.
- Added methods for updating the model, encoding, and decoding symbols.
- Included utility functions for reading from files and testing the algorithm.
- Verified functionality with various datasets to ensure accuracy.

This addition enhances the repository's collection of Python algorithms.

Author: LukOlen

compression/ppm.py

@@ -2,6 +2,7 @@
 import sys
 from collections import defaultdict
 
+#Description for the ppm algorithm can be found at https://en.wikipedia.org/wiki/Prediction_by_partial_matching
 
 class PPMNode:
     def __init__(self) -> None: