Changed the coarse grid file handling so that we don't keep the file open for the whole simulation.  This did not fix the segfault issue, but is probably a better pattern to follow anyway.
Spoke too soon with the last commit.  That commit, together with this one, seems to fix the segfault when writing the primary output.
Hooked up 4d interpolation for the enthalpy field
avoid segfault if we try to interpolate a bit beyond our time bounds, which might happen on the last step.
use the interpolated coarse model thickness for the ftt mechanism
refactored interpolation code, and took steps to make sure we are consistently employing it
start loopin
first version of fine->coarse feedback
Make sure we add a little padding for ourselves, and a safety net in case we try to access out of bounds.