Interface and simple implementation of fully implicit system solver.

This is done in preparation for adding a cpr-preconditioning solver
for the fully implicit black-oil system. The existing implementation
that concatenates the whole system and passes it to some linear solver
has been moved from a private function of FullyImplicitBlackoilSolver
to the class FullyImplicitSolverSimple.

To enable this decoupling, the residual struct has been copied out
of the FullyImplicitBlackoilSolver class and is now an independent
struct: FullyImplicitBlackoilResidual. The opportunity has been used
to replace the field mass_balance with material_balance_eq, which is
more precise.
Use new linear solver interface in fully implicit solver.

This includes:
 - Using the class FullyImplicitBlackoilResidual instead of
   in-class definition for the residual object.
 - Changing residual field name mass_balance to material_balance_eq.
 - Letting the simulator and solver classes accept a
   FullyImplicitSystemSolverInterface instead of a LinearSolverInterface.
 - In sim_fibo_ad and test_implicit_ad, instantiate class
   FullyImplicitSystemSolverSimple, replicating existing behaviour.