Forward declare PhaseUsage as struct, not class.
Merge pull request #83 from atgeirr/fix-warning-struct-class

Forward declare PhaseUsage as struct, not class.
Remove rs_or_sg_eq from the residual.

This has not been used since the introduction of live gas changed
the treatment of rs (and rv) values to make a more compact system.
Merge pull request #84 from atgeirr/remove-unused-residual-part

Remove rs_or_sg_eq from the residual.
Merge remote-tracking branch 'upstream/master' into opm-parser-integrate