add type hints

[plming]

I've added type hints for parameters and returns.
I referenced mostly docstring, implementation at few points.
It doesn't affect any runtime behavior and I checked all test passed!

[mthh]

Thank you! This is something I had thought of but didn't jump into!

Your type hints seem intuitively correct but when testing with mypy I have some errors. I think it's due to inconsistencies between some of my code and the documentation in the docstrings (which must have guided your additions).
I will just play a bit with the type hints and mypy to see if I merge it as it or if I take the opportunity to modify the docstrings and some of your hint types.

[mthh]

Is this OK if I push further modifications to your type-hint branch so that we can track it on the same PR ?

[plming]

Sure! Thanks for working :)

[mthh]

I changed a few things :

• the predict method is also working with a single scalar value,
• the group method returns a List of numpy arrays,
• most of the functions were documented as working with "an iterable sequence of number", which was misleading because they all the sequences are iterables, and most of the function are only working with Sequence (they dont work with something like (i for i in range(100)) expect the predict one), so I changed the Iterable typing to Sequence in the appropriate places.

Are you OK with my changes ?

[plming]

All good! But before we go, I fixed docstring with exact type hints. Please check it, and then fine to merge it!

[mthh]

Indeed, thanks!

And thank you for this useful contribution!

I will release a new version later today.