Switch PVT Evaluators to Use Explicit External Condition Interface

The criteria for whether the fluid is saturated or not is moved from the within the pvt calculations to the solver, and passed to the pvt calculations as arrays of opm-core's `PhasePresence` values.

The new criteria is set to `Sg > 0`, previously `rs >= rsBbp`. With this criteria the convergences problems 
is gone for the SPE9 simple case and moreover the overall number of iterations is reduced.

This update depends on OPM/opm-core#442.

This change incorporates, supersedes and closes #68.