Housekeeping/clean code

[CosmoMatt]

This is a general housekeeping PR to clean up the repo for the next (main) stage of the code development.

closes #31

• [x] Expands docstrings throughout
• [x] Standardises docstrings
• [x] Extensive debugging of all functions
• [x] Refactoring of various functions
• [x] Simplifying package structure (sub-module names etc.)
• [x] Add admissibility tests for directional filters