Pre-factorize local_solver

[jorgensd]

As an optimization at a later stage, we can pre-factorize the projection operator introduced by Johannes by adding:
https://bitbucket.org/fenics-project/dolfin/src/fd662efc1c0ddefa341c1dac753f717f6b6292a8/dolfin/fem/LocalSolver.h?at=master#lines-114:115
instead of solve_local_rhs/solve_global_rhs depending on the RHS.
We could then first assemble the lhs (in a class initialization), factorize the matrix once (if the mesh doesn't deform) and keep a cache of all the local element matrix factorizations.