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.
