renamed fgmax_interpolate.f90 to fgmax_interpolate1.f90 and added fgmax_interpolate0.f90 with constant rather than bilinear interpolation.  Suggest using the latter for future geoclaw runs.
Modified fgmax_values so max h is recorded rather than eta_tilde -- better choice for making sure proper flow depths recorded when topo changes.  Also modified fgmax_frompatch to computed eta properly for updating arrival times.  This requires changes to plotting routines.