rotation matrix for anisotropy functions

[landinjm]

This may be useful for multi-order simulations as a helper function to convert some generic anisotropy function to some angle.

The user would still have to specific what angles are available, but this would make it easier to implement these kinds of things.

[wband]

I ai'd some of those utilities for an abortive subproject back in 2023: https://github.com/wband/phaseField/blob/elastoChem/applications/elastoChem/customPDE.h.