Type annotations

[effigies]

We should add type annotations, particularly to such heavily-used functions and properties as DataobjImage.get_fdata(), SpatialImage.affine. This will make it easier on users working in environments that provide type hints and autocompletion.

[effigies]

Useful references:

• https://docs.python.org/3/library/typing.html
• https://numpy.org/devdocs/reference/typing.html

[effigies]

Mypy seems to be the way to get moving with type annotations. They've got some docs for Using mypy with an existing codebase.

[mgxd]

Here's a talk that is relevant: https://www.youtube.com/watch?v=wbohVjhqg7c

[mgxd]

Just writing down a thought I had:

I wasn't sure whether to use nibabel.filebasedimages.FileBasedImage or nibabel.spatialimages.SpatialImage as a general base for an image I want to load(). Perhaps there should be a nibabel.types module with some common types / basic information to help annotate code.