NAME:
  astcvt
 PURPOSE:   (one line only)
  Convert between different astrometric coordinate systems.
 DESCRIPTION:
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  astcvt,srcnam,srcval1,srcval2,info,dstnam,dstval1,dstval2
 INPUTS:
  srcnam - String that identifies the type of input data
              xy - raw image coordinates
              mn - normalized monolith coordinates
              sn - tangent plane coordinates (radians)
              SN - tangent plane coordinates (arcsec)
              rd - right ascension, declination (radians)
  srcval1,srcval2 - input pair of values (scalar or vector),
                       must have the same length
  info   - full astrometric support structure (see astinfo)
  dstnam - String that identifies the type of output data, same list as srcnam
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  dstval1,dstval2 - output pair of values
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2011/11/20 - Written by Marc W. Buie, Southwest Research Institute
  2020/01/23, MWB, force principle value for RA on rd output