new argument to ddm solve() to specify if the subdomain matrices don't change...

new argument to ddm solve() to specify if the subdomain matrices don't change when changing the physical/artificial commutator.

If matrices don't change, always reuse the factorization in solve() - which eliminates 2 factorizations per solve per subdomain.

This should eventually be auto-detected, by checking if the bilinear terms in the volume formulations depend on the physical/artificial commutator.

This patch also changes the handling of verbosity for "inner" solve operations, by setting to verbose-1 (instead of the harcoded value 2)