fix densities for new parser in BlackoilPvtProperties

that is one of the more subtle differences between the old and the
new parsers. now, valgrind does not seem to complain anymore, so everything
should be All Right (TM) ;)
Merge pull request #527 from andlaus/fix_blackoil_pvt_props_in_new_parser

fix densities for new parser in BlackoilPvtProperties