Fixes privar switch models when caching is disabled.
New state variable in the privar vector that indicates phase presence
The phase presence is no exactly where it's needed and also set by the user in the methods initial and dirichlet. The initialPhasePresence method is no longer needed.
The privar switch does not store the phase presence any longer
unfortunately we need to copy the solution vector when using umfpack and superlu because the interface are hardcoded to fieldvector as block type (this includes the default AMG solver using umfpack as coarse grid solver)