Improve the affine registration module

[brudfors]

Bugs:

• Currently, jittering the resampling grid introduces a small misalignment in the resulting registration (therefore disabled by default). Is something wrong with the implementation perhaps?

Enhancements:

• Work on 2D images, not only 3D.
• Ensure input shape is 3D, even if, e.g., a 4D tensor is given.

Features:

• Add more optimisers to _core._do_optimisation. Bayesian optimisation (BO) would be an interesting candidate.