OpmFind.cmake: prevent multiple calls to the same module in the same run

this prevents to check for the same module more than once in the same
cmake run and should thus speed things up a bit as well as make the
output a bit cleaner. For this I assumed that the ${name}_FOUND cmake
variable does not get cached, which it seems like it does not...
OpmFind.cmake: also check for CAPITALIZED found variables

thanks to @rolk for catching this
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 clearity
4. The new interface is tested in test_blackoilfluid with data from
liveoil.DATA and wetgas.DATA
Initialization using the RV keyword
The initial Rv value is specified using the RV keyword in the Parser.
The Rv values are used in the computation of the initial surface
volumes.
Change the extrapolation in the livegas pvts
The special extrapolation for pressure values above
the maximum table value is removed to sync with Mrst
Phase pressure is used for surface volumes calculations
Fixing comments
The comment now currently say 1/Bg
Merge pull request #462 from totto82/wetgas

Added pvt functionality for wetgas
Merge pull request #448 from andlaus/prevent_multiple_package_detections

OpmFind.cmake: prevent multiple calls to the same module in the same run