Add a Cholesky factorization for SPD ToepMat

[rluce]

A useful addtion would be a chol function for ToepMat. This would be based on the Schur algorithm. In addition, a cholsolve method should be implemented, that uses the Schur code with implicit triangular solve, resulting in a O(n) memory solver.

[rluce]

This could also be extended to LDU factorization, provided that the matrix is strongly regular.