The name 'H' can mean either the species by that name or the entahlpy
of the reactor, in the case of ConstPressureReactor, and the previous
behavior always returned the index of the enthalpy.
This changes the behavior to preferentially return the species, and
adds alternative names for reactor state variables that are less
likely to generate namespace collisions: 'mass', 'volume',
'int_energy', 'enthalpy', 'temperature', 'distance', 'velocity'. The
single character names are still supported.
Resolves Issue 193.
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