This PR lays some groundwork for further type annotations. The main thing it does is makeSpatialImage generic over its affine type, which can either beAffine (an alias for ndarray that might be refined in the future) orNone.

This is a somewhat pedantic thing to track, as it is extremely rare not to provide an affine to an image, but there are cases where it is the correct thing to do. This allows for a forthcoming PR to annotate functions that require an affine or can accept an image withNone in it.

It also introduces tests where we specifically test for whatmypy says the types of variables are, which lets us verify not only that our types are internally valid, but that they expose what we expect to downstream users.