Update docstrings, use numpydoc over napoleon

[simonbowly]

Addresses #66

• Use numpydoc over napoleon (seems to be cleaner, handles intersphinx properly)
• Update intersphinx mappings
• Add numpydoc aliases
• Update docstrings of (almost all) mods
• Update docstring of MVP mod (holding off to avoid conflicts with Change portfolio return type #84)

[rluce]

Not sure if it does more harm than help, but here is the torturous tool... https://www.pydocstyle.org/en/6.3.0/usage.html

[simonbowly]

Maybe later ... for now it's a wall of warnings, e.g.:

D401: First line should be in imperative mood; try rephrasing (found 'This')