Fixed a couple of bugs and added tests

When removing channels and thinning, the number of filters of the
next layer was not set correctly.

When loading a model that has already been thinned (e.g when loading
a model, thinning, saving, loading), don’t crash on wrong tensor sizes.

Cache the thinning recipe in the model when loading from checkpoint.
Without this, a loaded thin model will lose its recipes when saved to
checkpoint