 combining the <code>red</code>, <code>green</code>, <code>blue</code> and
<code>nir</code> bands according to the formula provided at
<code>https://lta.cr.usgs.gov/terralook/what_is_terralook</code> (a
website that worked once, but failed as of Feb 2, 2017), namely
that the <code>red</code>-band data are provided as the <code>red</code>
argument of the <code><a href="https://rdrr.io/r/grDevices/rgb.html" class="external-link">rgb()</a></code> function, while
the <code>green</code> argument is computed as
2