Migrate all doc strings to use numpydoc

[taldcroft]

Description

This migrates all the docstrings for xija to use the numpydoc style / extension. The actual docstring migration was done fully automatically with the statement below after doing a --user local install of pyment with pip.

~/.local/bin/pyment --output numpydoc --write --convert xija

There is room for improvement in the actual values of parameters etc., but this PR is just about migration.

Testing

• [N/A] Passes unit tests on MacOS, linux, Windows (at least one required)
• [x] Functional testing

Functional testing

Built HTML documentation with no warnings. The API docs appear as expected.