FullyImplicitBlackoilSolver: make constantState() and variableState() consistent

With this, constantState() just calls variableState() and throws away
the derivatives. This might be a bit slower than necessary, but it
makes these two methods automatically consistent and constantState()
is only called once per timestep anyway...

thanks to @atgeirr for the suggestion!
Merge pull request #108 from andlaus/FullyImplicitBlackoilSolver_unify_state_conversion

FullyImplicitBlackoilSolver: unify the state conversion routines