Update docstrings across metasyn

[Samuwhale]

Currently it states that to_dict must be implemented in derived classes from Base (distribution), but that is not the case. It should be changed to state that _param_dict , default_distribution and _param_schema are required instead.

In general, it would be nice to improve the docstrings - this will also make the automatically generated API on the readthedocs easier to understand.

• Update docstring in Base distribution (fixed in Improved API and Developer guide #220)
• Update module level docstrings (fixed in Improved module/package docstrings #226)
• Update class level docstrings
• Review and update function level docstrings (I think these are in a relatively okay at the moment, but for 1.1 they should be reverified)