deprecate rotation_matrix_* functions

[CagtayFabry]

I think we can deprecate the transformations.rotation_matrix_* functions in favor of WXRotation.as_matrix to allow more input options and clean up the API a bit

They are mostly used for testing

[marscher]

For the aid of deprecating funtions, at least in my experience, a deprecation class/function decorator is very useful. It shows a warning indicating that the called object is deprecated and can be turned into a an error via the warnings module to clean up all usages within the code base. Users will just see the warning for a couple of versions and can adopt their code accordingly.

Back to topic, I agree that these functions are a indeed a duplication of features and should be replaced by scipy Rotations.

[vhirtham]

Since we are the only users so far, I think we can just remove it. However, having a deprecated decorator is probably a nice thing for the future. I'll add one...

[CagtayFabry]

yes this should only "break" internal code so removing should be fine :)
we can redefine these in the tests directly to simplify refactoring