Radiative transfer routines
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radtrans_sfcsolz            computes solar zenith angle at surface given location and time
+- radtrans_localvec        creates arrays of up, north, and east vectors for fixed locations
+- radtrans_sunmod          computes solar zenith angle from date, time and location vector
   +- radtrans_sun2000      computes the Sun vector in geocentric inertial 
   |  +- radtrans_jd        converts calendar date to Julian day
   |  +- radtrans_ephparms  computes ephemeris parameters used by other Mission
   |  +- radtrans_nutate    computes the nutation in longitude and the obliquity
   +- radtrans_gha2000      computes the Greenwich hour angle in degrees for the
      +- radtrans_jd        ./.
      +- radtrans_ephparms  ./.
      +- radtrans_nutate    ./.

radtrans_sfcirr    computes transmitted part of irradiance just below sea surface
radtrans_sfcrmud   computes average cosine for direct irradiance

radtrans_radmod    Aas/Ackleson radiative transfer model for one grid cell/wave band