Add 'colormap_tag' keyword argument to geotiff writer

[djhoese]

Makes the functionality added to trollimage in pytroll/trollimage#99 available to Satpy. See that PR for more details.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2028 (09877b8) into main (a3fa4c4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2028   +/-   ##
=======================================
  Coverage   93.65%   93.65%           
=======================================
  Files         282      282           
  Lines       41878    41878           
=======================================
  Hits        39220    39220           
  Misses       2658     2658           

Flag	Coverage Δ
behaviourtests	4.76% <ø> (ø)
unittests	94.18% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/writers/geotiff.py	93.22% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a3fa4c4...09877b8. Read the comment docs.