Implement FkRotation as subclass of Rotation by rewriting using a rotation tensor

[APJansen]

Implements an old TODO by @scarlehoff to write the FkRotation layer as a subclass of Rotation.

Unlike for FlavourToEvolution the rotation matrix is fixed, so I thought it made sense to define it right in the class.

Unrelated to this, the docstring isn't accurate, referring to 8 dimensions while actually there are 9. This may be a good occasion to fix it, but I'm not sure what to write as I don't know where the +1 is from, any suggestion?

I've verified that this gives the same output using the script below, and also checked that the new way is about 3-5 times faster.

rot = FkRotation()
test_x = tf.random.normal(shape=(1, 20, 9))

out1 = rot(text_x)
out2 = rot.call_old(test_x)

print(out1 - out2)

[scarlehoff]

Unlike for FlavourToEvolution the rotation matrix is fixed, so I thought it made sense to define it right in the class.

Yes, indeed. And in any case, it is better than the combination "by hand" that we had right now imho. Thanks!

Unrelated to this, the docstring isn't accurate, referring to 8 dimensions while actually there are 9. This may be a good occasion to fix it, but I'm not sure what to write as I don't know where the +1 is from, any suggestion?

The +1 is coming form the addition of V15 to the fit, in order to allow for the charm-anticharm asymmetry a few months ago. During that change it was anticipated that some "8s" would need to be changed by hand, I'm happy that it's only docs for now :P #1629 (comment) so please update the docstr!