Fix oil injecting behaviour of TransportSolverTwophaseReorder.

This should fix issue #398. My testing now indicates correct behaviour of all two-phase transport solvers in opm-core with respect to injecting oil instead of water (also for opm-autodiff:TransportSolverTwophaseAD).

However, the reports written to the terminal stating amount of injected oil are still wrong. This is not as critical as correct solver behaviour, however. I will post this as a separate issue.