simplesteamaircao2h2.hh and steamn2cao2h2 are not tested at the moment

@heck and I realized while implementing molarDensities that simplesteamaircao2h2.hh and steamn2cao2h2.hh are not used in any tests we could find on master. Is that correct?

Should we still keep it? If yes, we propose to use it in a test and also implement the molarDensity() in simplesteamaircao2h2.hh as it is done in !498 (merged).