[bugfix] Fix difference order and correctly transform to tangent coords

While the translation part in the difference method computes 'a-b', the rotation part computes 'b-a' (which you get when calling Rotation::difference(a,b)) Furthermore, the Rotation::difference method returs a skewsymmetric matrix, which has to be transformed to yield a tangentvector in the tangent space of 'a'

mentioned in commit e1172c15

It's been a long time...

I never merged this because I didn't want to merge it without a unit test. Now I finally got around to writing such tests. These are in !93 (merged), and as it turns out the buggy difference method should be replaced by log, which is what most other manifold implementations have.

Therefore, !93 (merged) deprecates the difference method. I decided not to fix it, because if people out there do use this (no code in dune-gfe apparently does), I didn't want to tacitly change the behavior after so much time, shortly before deprecating the method.

closed

mentioned in commit c20c7471