gh-358: add static types support

[paddyroddy]

Closes #358. Replaces #308.

This PR adds static type support, hence satisfying mypy. With the adaptation of #67 I anticipate typing to change, but it's much easier to do once mypy is passing initially.

The only # type: ignore[] comments remaining are attr-defined which are all attributed to np.trapz, those can easily be fixed by #207.

I have turned off:

• disallow_untyped_decorators this is being triggered by pytest and nox. The equivalent # type: ignore[] error is misc which isn't the most informative
• warn_return_any which is due to external libraries like cosmology - this could be revisited in the future

I have decided to re-write some of the tests which were simply not following the typing - like passing is a list rather than a np.ndarray. The alternative approach would be having really messy typing.

I'm not expecting this to be fully perfect. We will likely have to iterate on this, and completely change once we follow #67. But this is a good first step.

Do make sure to look carefully. Hard to avoid such a big PR with this. There were over 200 # type: ignore[] comments to fix!

[paddyroddy]

@ntessore I've addressed through your comments

[ntessore]

Fantastic! Well done!

[Saransh-cpp]

Awesome! Thanks for putting so much efforts into this, @paddyroddy!

[paddyroddy]

Thanks both for the review. Will merge this and update the downstream PRs.