Transform: fix copy/paste error

Ahem, vectors are translation invariant but I messed up the
implementation for the Transform-Vector multiplication. That's where
unittests come in handy.
Transform: we don't need Transform::apply_inverse

T.apply_inverse(v) is semantically the same as inverse(T).apply(v). Get
rid of the unnecessary code then.
tests: add tests for Transformation

We now should have test coverage for the whole codebase. For the
Transform tests I rely a bit on implicitly tested properties. I hope
this doesn't come around one day.