Added pvt functionality for wetgas

The pvt interface is extended to handle wet-gas systems:
1. rvSat is added as a function in the PVT interface
2. SinglePvtLiveGas computes the pvt values and its derivatives
3. The old rbub variable is changed to rsSat for clarity
4. The new interface is tested in test_blackoilfluid with data from
liveoil.DATA and wetgas.DATA
5. Initialization using RV is added 
6. The extrapolation of the livegas properties is changed to mimic Mrst
7. Phase pressure is used in the computation of the surface volumes in the
initialization.