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 branch 'FullyImplicitBlackoilSolver_unify_state_conversion' of https://github.com/andlaus/opm-autodiff into test_andlaus

Conflicts:
	opm/autodiff/FullyImplicitBlackoilSolver.cpp

Resolved by using the new, reduced version of constantState().