Add decision variable analysis notebooks and update binning utilities

- Added decision_variables_extraction.ipynb: main analysis notebook for extracting DVs from neural data
- Added decision_variables_methodology.md: documentation of approach and theoretical background
- Added dv_plan.md: structured plan for implementing GLM-based decision variable extraction
- Updated PCA notebook with exploratory data analysis and code comments
- Enhanced binning.py with improved type hints and error handling

Author: HAM41