NAME:
  craterfun
 PURPOSE:   (one line only)
  Compute depth as a function of distance from center for a crater
 DESCRIPTION:
  Based on equation 12 from Statler, Icarus, 202, 502-513 (2009).
 CATEGORY:
  Miscellaneous
 CALLING SEQUENCE:
  res=craterfun(r,d)
 INPUTS:
  r - distance from crater center (same units as d)
  d - crater diameter (arbitrary units)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  delta - width of the raised rim (default = d/20)
  q     - depth to diameter ratio (default = 0.2)
 OUTPUTS:
  crater depth (same units as d)
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2018/03/15