black oil model: simplifiy the intensive quantities class

this can be done using the 'saturatedOilGasMoleFraction()' method of
the black oil fluid system.
reservoir test problem: change the reference solution

this became necessary after the recent correction of how the Rs value
is considered...
black oil vtk module: use the correct name and description for the gas dissolution factor
black oil VTK module: add possibility to write out the mass fraction as Rs value

this simplyfies comparisons with the deck...