Refactor Features subpackage

[bvarjavand]

Reformat the code in base.py, neighborhood.py, and linear_features.py. Additionally, refactor and overhaul test_subneighborhood.py according to the standards mentioned in the Refactor project.

This means:

• typing hints on every method and class
• google-formatted docstrings on every method and class
• input checking (TypeError and ValueError) on every input for every method
• tests on each method validating inputs
• tests on each method validating returns
• docstring on each test describing what it does
• An example method which has been refomatted.
• An example method which has not been reformatted. Note the lack of type hints and type checking.
• An example which lacks documentation.

Current progress in in the feature-rename branch