Splits ConservedVars into single gas and multi-component gas versions.

This change removes the empty species_mass array for non-mixture states, and introduces a mixture-specific CV structure called MixtureConservedVars. MixtureConservedVars derives from ConservedVars and adds the species_mass array. The change is transparent to most fluid components, but obviates a good deal of special-case processing needed to deal with a ConservedVars that carries around an empty/unused array, at the MIRGE-Com level and below.

fixes: #575

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• Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• Does the implementation do what the docstring claims?
• Is everything that is implemented covered by tests?
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