Specialized well state class for the fully implicit blackoil solver.

This is part of a larger change to the well formulation of the fully implicit blackoil solver.

The fully implicit solver needs to communicate the well perforation flows per phase. The current WellState class contains the perfRates() member, which only has total rates (not per phase). Instead of expanding this class further and updating other clients of WellState to set the new member, which not all can do, I have chosen to create a new class.

The new class WellStateFullyImplicitBlackoil contains a WellState object, and can grant read-only access to it, for functions that require such an object.