NAME:
  snrpred
 PURPOSE:   (one line only)
  Compute an estimated signal-to-noise ratio for a point source
 DESCRIPTION:
 CATEGORY:
  Photometry
 CALLING SEQUENCE:
  snrpred,system,smag,omag,snr
 INPUTS:
  system - anonymous structure with the following tags:
             jy0mag:   Janskys for zero Vega magnitude at lambda
             lambda:   center wavelength [nanometers]
             topt:     optical surface temperature of telescope [K]
             emis:     net emissivity to sky
             rdnoise:  read noise [e-]
             aper:     telescope aperture [cm]
             fnum:     f/# at focal plane
             bandpass: bandpass of instrument [nanometers]
             pixel:    size of pixel [microns]
             sky:      sky brightness [mag/sq arcsec]
             exptime:  exposure time [seconds]
             dark:     dark signal [e-/sec/pixel]
             tput:     throughput, central obs, reflectsion and QE
             seeing:   FWHM arcseconds
  smag - Magnitude of the occultation star source (at lambda)
  omag - Magnitude of the occultating body (at lambda)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  VERBOSE - if set will generate lots of printed output.  This is rarely
              useful if "smag" is a vector.
 OUTPUTS:
  snr - Estimated signal-to-noise estimation per point
 KEYWORD OUTPUT PARAMETERS:
 CONFIGURATION:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2022/02/04 and
     based on a spreadsheet provided by Mike Skrutskie, UVa.
  2022/02/22, MWB, added object magnitude and scintillation estimates