Add massFraction function to Media library

[dzimmer]

This is a second pull request replacing #3825 which attempted to resolve #3650.
#3825 should be discarded and archived consequently.

This pull request is based on the most recent master of the MSL and I have taken the critic of #3825 into account:

• The comments now speak of "mass fractions" instead of "mass fraction" in order to be consistent with the otther comments
• I have extended the examples of ModelicaTest to include calls of this function at 3 examples to cover the pureSubstance case as well as the reduced and non-reduced Mixture case.

The added function is till called massFraction not massFractions for the following reasons:

• to be consistent with the outputType
• to be compatible with the ThermoFluidStream implementation already using this function
• to be compatible with the XRG extensions to their media where they have implemented it.

The following changes are included in the pull request.

• implemented massFraction for partialPureSubstance
• implemented massFraction for PartialMixtureMedium
• added 3 test cases for ModeliceTest covering pure / reduced mixture / non-reduced mixture

[CLAassistant]

All committers have signed the CLA.

[casella]

LGTM