A simple carbon detonation.  The reaction network should be set through the
GNUmakefile.

This sets up a domain with a uniform density (dens).  A large
temperature (T_l) is placed in the left side of the domain, and an
ambient temperature (T_r) is in the right.  The parameter center_T
specifies where, as a fraction of the domain length, to put the
interface, while width_T determines how wide the transition region is,
as a fraction of the domain length. Finally, the parameter cfrac is
the initial C12 fraction (the remaining material is O16).

The left side and right side can optionally approach each other with a
non-zero infall velocity, vel.

When run, the large temperature in the left side instantly flashes the
C fuel, generating a lot of energy and a large overpressure.  The
signature of a propagating detonation is that a narrow energy
generation zone will keep pace with the rightward propagating
shockwave (as seen in the pressure field).  

Note: if the domain is too small, then the burning will decouple from
the shock wave, and you will not get a detonation.

