An implementation of the Homologous Dust Collapse, a 'pressure-less'
configuration that collapses under its own self gravity.  An analytic
solution that describes the radius of the sphere as a function of time
is found in Colgate and White (1966).  This problem is also found in
the FLASH User's Guide and Mönchmeyer and Müller (1989).

A large density, rho_0, is placed inside a sphere of radius r_0.  The
pressure should be negligible, formally, p_0 << 4 pi G rho_0^2 r_0^2 /
gamma (see Flash Users Guide).

It is not clear what value of p_0 different papers use.

From the FLASH User's Guide, we have:

  rho_0 = 1.e9 g/cc
  r_0   = 6.5e8 cm

and a typical timescale is ~ 0.06 s.

The diagnostic routines to create radius-vs-time plots from the 1d, 2d and 
3d plotfiles are in Castro/Diagnostics/DustCollapse.

