H2O-N2 fluid system: some cleanups and corrections

- extend some comments
- assume an ideal mixture for gas everywhere, i.e. use the partial
  pressure to calculate the "partial" specific quantities like
  e.g. the specific partial enthalpies of water and nitrogen
- let the water and nitrogen component determine the "partial
  densities" of gas if the complex relations are enabled. before, an
  ideal gas was assumed. (now the individual components can be
  non-ideal gases, but we still assume an ideal mixture.)

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