TODO

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Probably the sun position calculation should be replaced
with

 G_calc_solar_position()

currently used in r.sunmask. G_calc_solar_position() is based on
solpos.c from NREL and very precise.

MN 4/2001

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Update
  https://grasswiki.osgeo.org/wiki/R.sun

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Fix https://trac.osgeo.org/grass/ticket/498

pseudo-data test-case
https://trac.osgeo.org/grass/ticket/498#comment:22
#spearfish (further north than NC so more defined shadows)
g.region -d
r.mapcalc "undulates = (2 + sin( row() * 2 ) + cos( col() * 2 )) * 500"
r.colors undulates color=bcyr
nviz undulates

DAY=355
time r.sun elevin=undulates day=$DAY step=0.05 \
  beam_rad=rad_test.und.355.beam \
  diff_rad=rad_test.und.355.diff \
  refl_rad=rad_test.und.355.refl
