NAME:
  astpdf
 PURPOSE:   (one line only)
  Compute a discrete numerical probability sample for astrometry
 DESCRIPTION:
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  astpdf,ra,dec,sig1,sig2,sys1,sys2,pang,pdf
 INPUTS:
  ra - right ascension of the position [radians]
  dec - declination of the position [radians]
  sig1 - major axis of error, gaussian [arcsec]
  sig2 - minor axis of error, gaussian [arcsec]
           by convention, sig1 >= sig2
  sys1 - systematic component along major axis [arcsec]
           If provided as a scalar value, the range is +/- of that range
           about zero.  If provided as a two-element vector, this gives
           the positive and negative limit (as signed numbers).  For example,
             [-1,2] means the range is from -1 to +2.  The probability is
             computed as a uniform distribution over the range.
  sys2 - systematic component along minor axis [arcsec]
           This input works just like sys1.  The relative magnitudes of sys1
           and sys2 are arbitrary.
  pang - position angle of the major axis, measured eastward from north
           [radians].  This value should be treated over the range of [0,2pi)
           because the positive component of the systematic distribution of
           the major axis points in this direction.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  NSAMPLES - number of samples in the distribution (default=10000)
 OUTPUTS:
  pdf - 2D array of ra,dec values dimension [N,2]
 KEYWORD OUTPUT PARAMETERS:
 CONFIGURATION:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2024/01/02