Cooling working with Physical Units
Split cell_source into background-specific files.  Added GRDISC.
Gammalaw EOS implemented.
GRDISC almost done, cleaned up several unused variables.
GRDisc implemented.
eos_cool works with GRDisc.
Fixed typo in flux_GRdisc, implemented eos_gasrad.
Gasrad sound speed uses correct formula.
Turned viscosity back on.