Optimize Variational non linear solvers

Solves the following issue(s):

Closes None

Core changes:

Small update of the preconditioners to add the domain attribute.

Model-specific changes:

Use a "hand made" preconditioning for the solver of the system in the Newton iteration by taking advantage that the Jacobian is mostly a mass matrix for which we already have good preconditioners. This simple change allow for roughly 80% speedup as most of the time (90%) was spent on GMRES solve.

I also tested and I can now use larger time steps than with Picard iteration.

Documentation changes:

None