This creates a single implementation of the calculation of the contributions of
walls and surface chemistry to the governing equations for all reactor types.
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.
This formulation of the reactor governing equations, with temperature as a state
variable, works better for ideal gas mixtures. This way, most of the Jacobian
components are derivatives at constant temperature, eliminating the need to
recompute the temperature-dependent part of the rate expressions when computing
these entries.
