fix more (hopefully all) conversion factors

also, correct the dimension of the Rv item of the PVDG keyword which
is 1/Rs. Finally, the conversion factor for stock tank barrels to m^3
was off by a factor of 1000. (I suppose that liters instead of m^3
were assumed to be the SI unit in this case.)
Merge pull request #83 from andlaus/fix_more_conversion_factors

fix more (hopefully all) conversion factors