BrineCO2 Fluidsystems uses wrong/misleading gasDensity_ calculation

The brineco2.hh fluidystems uses an incorrect gasDensity calculation in the private gasDensity_ function. It returns the unchanged CO2 gas density, while the density function implies a lot more complex density calculation similar to the liquidDensity_. This seems to be like this from the very first commit on git, but e.g. in a biofluidsystem.hh that was derived from the brineco2.hh a long time ago by @hommel on dumux-devel there is a more complex gasDensity calculation using Dalton's law available. (see below) I suggest we change/fix this once most tests pass, because this might change some results for the next branch. I am not sure, what we should do about the master (or 2.12).

Additionally the normalization of some moleFractions etc. seems a little sketchy to me, but that might be necessary. Additionally the second the brineco2.hh still uses the deprecated xl and xg naming. This should be changed, while fixing this.

static Scalar gasDensity_(Scalar T, Scalar pg, Scalar xgH2O) { Scalar pH2O = xgH2O*pg; //Dalton' Law Scalar pCO2 = pg - pH2O; Scalar gasDensityCO2 = CO2::gasDensity(T, pCO2); Scalar gasDensityH2O = H2O::gasDensity(T, pH2O); Scalar gasDensity = gasDensityCO2 + gasDensityH2O; return gasDensity; }