Add elastic constants tools

This code is from my elastic constants generator
scripts which can be found at:

    https://github.com/andreww/elastic-constants

These should probably be broken out into a seperate
libary to do "MSAT" in Python at sone point. The only
novel thing about these functions is the handling of
errors (including propogation through the matrix
inverse).
Make the plot colour particles by anisotropy.