Add SEVIRI + NWC SAF GEO VIS/IR cloud overlay composite

[pnuu]

This adds a composite that uses NWC SAF GEO cloud type data to add varying levels of transparency to a 24 h VIS/IR composite. Example image below.

[codecov]

Codecov Report

Merging #1659 (b917a31) into main (05c033e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1659   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files         258      258           
  Lines       37948    37948           
=======================================
  Hits        35161    35161           
  Misses       2787     2787           

Flag	Coverage Δ
behaviourtests	4.83% <ø> (ø)
unittests	92.79% <ø> (ø)

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[mraspaud]

LGTM, thanks a lot for contributing this nice composite!