CholmodSolver: Allow to reuse a factorization

Computing Cholesky factorizations is expensive, and they absolutely
have to be reused if the same linear system is solved with several
different right hand sides.  The previous implementation of
CholmodSolver wouldn't allow this: The 'solve' method factorized
and solved together.

This patch makes the interface of the CholmodSolver class richer
to allow for factorization reuse.  It is now possible to set the
matrix, rhs and solution storage separately.  The matrix can be
factorized by calling a new 'factorize' method, and if 'solve'
is called afterwards, the matrix won't be factorized again.

On the other hand, when calling 'solve' by itself the 'solve'
method will do the factorization.  In other words, the patch
is fully backward-compatible.