Function docstring improvements

[brandynlucca]

Descriptions for many functions (e.g. calculate_aged_biomass) need to be improved so they include additional information such as the expected data outputs and, in the case of dict and DataFrame objects key column names and variables that are added that will be relevant for other calculations or results the users have generated.

[brandynlucca]

Documentation for the user-facing methods (i.e Survey-class methods) are being addressed in #297. While general improvements to docstrings are warranted throughout the library, this will be addressed on a case-by-case basis and will likely be better addressed once many parts of the library have been refactored with new tests (#245 and #263). Therefore, this will be closed once #297 has been merged.