Override gr_mat_eigenvalues

[fredrik-johansson]

Currently gr_mat_diagonalization can be overridden by rings but gr_mat_eigenvalues just has a default algorithm.

[fredrik-johansson]

Two related problem which I'm too lazy to create a new issue for right now: gr_mat_diagonalization currently computes the R matrix by linear solving, then inverts R to obtain L. It is probably better (in terms of numerical stability or intermediate expression swell, where applicable) to do an independent linear solve for L. Also, the calculations can be simplified when all eigenvalues have multiplicity 1.