Refactor `AffineTransformation`

[padix-key]

biotite.structure.superimpose() currently returns an AffineTransformation. Multiple aspects could be refactored here:

• As AffineTransformation represents actually a rigid transformation, it would be renamed to RigidTransformation
• Currently it uses two translations and a rotation. However a rigid transformation can always be represented by a single translation and rotation, which would make apply() faster
• It would make sense if other function also return a transformation instead of already transformed coordinates:
  • align_vectors()
  • orient_principal_components()
• Add ability to 'compose' transformations:
  • __mul__() for combination of transformation
  • __neg__() for to reverse a transformation