Add typing annotations

[cako]

Typing annotations are quickly becoming ubiquitous in Python. They improve readability, documentation, and when paired with a modern IDE, can improve autocomplete suggestions and supply advanced warnings.

I propose that we incorporate type hinting as much as possible in a future release, at the very least for function/class signatures in user-facing code core library code.

Definition of done

1. Type hinting in all user-facing function and class signatures. Anything which is exported should be type hinted.
2. Type hinting should use either standard library hinting (native in Python 3.8 and later, provided by the typing-extensions package in earlier versions) or NumPy typing when appropriate (provided under numpy.typing in NumPy versions 1.20 and higher). For example, a function which takes a numpy.ndarray should be annotated with numpy.typing.NDArray.

[cako]

@mrava87 I put this as a To do for v2.0.0, I hope that's ok!

[mrava87]

Sounds good! It is a lot of work but worth for a big release :) we can aim for it in coming months also to remove the various deprecated parameters here and there

[cako]

@mrava87 could we upgrade to NumPy 1.20?

[mrava87]

I guess you mean as minimal allowed numpy version?

For v2, sounds like a good idea :)