Comply to WMO RGB guidelines

[mraspaud]

The RGB composites defined in the WMO guidelines should be implemented is satpy.

[gerritholl]

That was my aim with #2670 but that one was closed as completed.

[pnuu]

#2670 was about EUMeTrain RGBs, not the newer WMO definitions.

[gerritholl]

WMO Guidelines

Note that:

• The WMO Guidelines use British English, satpy has so far used American English. This affects True Colour, Simple Water Vapour, Differential Water Vapour, and GeoColour.
• We may want to have aliases for RGBs that include filename-unfriendly characters and retain filename-friendly versions:
  • All RGB names except Airmass contain whitespace, none of satpy composite names do.
  • One (Volcanic Emissions (SO2) RGB) contains parenthesis. We might want to define a parenthesis-free alias.
  • All WMO RGB names are capitalised. Most satpy composite names do not contain capital letters (exceptions: highlight_C14, C14_flash_extent_density, C14_yellow_lightning, but those do not correspond to any WMO standard names).

Strictly speaking, there is no technical necessity for any of those differences. However, our composite names often end up in filenames, and users handling filenames in shellscripts may prefer if those filenames do not contain whitespace or parenthesis.

[djhoese]

@gerritholl Note that I've already done some of these changes for ABI:

#3205

In the slack and related issue discussions it was decided that the "read between the lines" name for the Volcanic Emissions (SO2) RGB was just "volcanic_emissions".