Use type hinting with mypy

[cwmeijer]

It is getting a bit convoluted what the input types of specific functions are. We now have options: numpy array, xarray, string, list of strings. I think it would help both development and users to specify at certain places what types we expect.

[egpbos]

Indeed, probably all the places where we currently feel that we have to specify it in the docstring (input arguments like matrix (np.array): blabla) we should have it as a type hint instead.

[elboyran]

Seems important as we are adding soon more modalities.

[stefsmeets]

We discussed this today in the retrospective. We agreed this is a good idea to add gradually.

My suggestion would be to just enable mypy and then see how it goes.

@geek-yang @cwmeijer