[Diversity module] Add Formulas to Docstrings, Add Tests, & Polish Module

[FarnazH]

See: #121

For each function (mentioned in #121), make sure that:

• implementation is clear with comments.
• docstring is polished and gives the formula implemented (add a reference to the paper used for implementation)
• tests have 100% coverage

[AWBroscius]

While trying to write documentation for this module, these are some of the implementation errors I ran into:

• the logdet function, as currently implemented according to this source, requires a features x mols matrix, while elsewhere in the package our standard is a mols x features matrix. Do I change the implementation or just note this in the docs?

• shannon_entropy() is not correctly implemented, and does not match the cited paper/equation. There is a comment to add a raise error, but reading through the cited paper I see no reason why there would be a value error in its calculation. Do you want me to rewrite this function?

• total_diversity_volume is also not implemented according to its paper. The function currently returns the degree of overlap between molecule neighborhoods, not the total diversity volume. I think we should rename the function instead of altering the implementation, as in the paper, total diversity volume does not describe a sub-set of data points, but the entire input space.

• The Wasserstien distance function wdud() needs to be rewritten

[FarnazH]

Post - PR #135:

• 93% Test Coverage (missing lines are 75-78, 240, 249, 314, 319, 384)
• Fix formula https://github.com/theochem/DiverseSelector/blob/main/DiverseSelector/diversity.py#L86
• gini_coefficient: Rename argument name a to x to be consistent with other functions
• hypersphere_overlap_of_subset: Rename lib and x into x and x_subset
• Remove redundant arguments https://github.com/theochem/DiverseSelector/blob/main/DiverseSelector/diversity.py#L57
• total_diversity_volume was renamed, but it was not changed everywhere. The tests fail as a result of this! https://github.com/theochem/DiverseSelector/blob/main/DiverseSelector/diversity.py#L45