Adding method/members to register postprocessing quantities

This is the beginning of the PostprocessedQuantities refactor.
Check for existing quantity
Use make_pair instead
Need to alert CachedValues we're going to have a quantity
BUild up map between local indices and libMesh variable indices
Infrastructure for Physics to register PostProcessedQuantities

Time to start cleaning up and having each Physics use this instead.
Want register postprocessing to have access to GetPot input
size() for CachedValues
That was supposed to be quantity_name_index_map
Need to assign system pointer
Need to swap the order of the map

Need to be able to search the map based on the VariableIndex, i.e.
the component coming from libMesh
Need to init_context with our new version
We can only postprocess scalar quantities.

Thus, get rid of a bunch of unnecessary stuff and redo register_quantity
API.
Adding compute_postprocessing_quantity

This is the API we'll move to to have each Physics compute their
postprocessing quantities. I don't really see there being much gain
with dealing with all the Cache shit vs. just evaluating it at each
point. If the repeated calls to each Physics pops up in timing, then
we can revisit. But this is much simpler code.
compute_postprocessing for LowMachNavierStokes
compute postprocessing for VelocityPenalty
omega_dot interface wasn't using R_mix

So got rid of it. Also, added "raw" method that takes T, rho,
and Y directly. This is in support of postprocessing refactoring.

Forgot to update test

Need to pass T by reference

This has to do with the caching of the temperature for the Antioch
interface.
Additional API for species enthalpy

Need this for postprocessing refactoring
Index variables for reacting flow postprocessing
Compute postprocessing quantities for reacting flow