Create invertmatrix.h

Create invertmatrix.h

Named matrix-inversion routines less cryptically. "ur0" meant that the
upper-right triangle was all zero, because the matrix was a
well-formed box-vector matrix, per docs in vec.h. Added unit tests and
Doxygen. There's not actually a maths module yet, but it's better
than it used to be.

Change-Id: I4fe53acef07c6710c3a6ee49fc344d312694cf2c