type hints + mypy

[drbenvincent]

Add type hints and run mypy.

• Fix numpy type hint here

  CausalPy/causalpy/plot_utils.py

  Line 11 in d74a5a8

  x: Union[pd.DatetimeIndex, np.array],

[Padarn]

Hyy @drbenvincent I'd be happy to lend a hand here if you like. I assume this issue is meant to cover adding mypy testing?

Fix numpy type hint here

Presumably to use nptyping?

[drbenvincent]

Hi @Padarn. Sure, I've not used mypy so I was going to learn that and improve the type hinting. But if you wanted to submit a PR, that would be great. It probably makes sense to add some short usage instructions in CONTRIBUTING.md as well.

My comment about numpy typing is a little cryptic, but I think I must have meant to add shape information (if required). But perhaps that's optional.

[Padarn]

Haha got it. Sure let me take a stab at it.