Matrix inversion

[cbouilla]

we should add mzd_inv(A) as a shorthand for inverting an invertible matrix.

[cbouilla]

malb said: "Ironically, M4RI does not have an implementation of asymptotically fast and in-place matrix inversion. However, once TRTRI is implemented, we can implement this easily by computing PLE + 2x TRTRI."

[cbouilla]

I think that inversion of triangular matrix is implemented (with reduction to multiplication).