MoistAir entropy and isentropicExponent issue

[RaphaelGebhart]

Added assert(relativeHumidty<=1) to MoistAir.isentropicExponent and MoistAir.specificEntropy and added 3 Test Models for further discussion closes #169

[dzimmer]

What is the reason for this being a draft? I would otherwise go on with the regression test

[RaphaelGebhart]

I think jakub suggested the PR to be a draft, because i called the package "MoistAirDiscussion" and wrote "I assume" and "I suggest" in the docu, since im no expert in the media model but just noticed some issues at saturation where the media model retuns not reasonable results without warning the user, and maybe also since we said we might want to tell the MSL directly.