aligning non-image layers

[thanushipeiris]

This PR should address #42 @jni

I just changed two lines - converting the input layer types to the more general napari.layers.Layer instead of napari.layers.Image. It seems to have worked.

Image to shape affine transform
https://user-images.githubusercontent.com/54516770/151113341-4f5185e9-ba83-4344-961b-04d29346c14f.mp4

Image to image transform (is preserved)
https://user-images.githubusercontent.com/54516770/151113418-b21b5c1d-27b6-4c74-aea4-c9353140cdd9.mp4

Note that I'm not actually trying to match anything in these examples! So they look weird. But the affine makes the expected distortions to the moving image.

[jni]

Thanks @thanushipeiris! I'll merge as soon as tests pass.

Ideally we would update the tests to include testing different layer type combinations, but, given #46 changes the tests and still needs to be merged, I suggest we do that after that gets merged. I'll create a new issue for it.