Add WellStateFullyImplicitBlackoil class.

This is intended to be used instead of the WellState class in the fully
implicit blackoil simulator. It contains a WellState to reuse the init()
method and to enable users to call functions requiring a WellState.
This is done with containment and an access member function,
basicWellState(), instead of with inheritance to minimize surprises.
Use WellStateFullyImplicitBlackoil class instead of WellState.
Merge pull request #102 from atgeirr/separate-wellstate

Specialized well state class for the fully implicit blackoil solver.