WMO Instruments Part 2.2: File Handlers

[sfinkens]

Update all file handlers to provide WMO instrument names. To be merged into #3390. This is the biggest one.

Summary:

• Add Enum with all WMO instrument names (satpy._instruments.OSCAR).
• Replace "sensor" attribute with WMO "instruments" in all file handlers.
• Deprecate "sensors" attribute in readers/*.yaml in favour of "instruments".
  • YAML reader: If "sensors" is found in the YAML, convert to "instruments"
    and print deprecation warning.
  • readers/*.yaml: Renamed "sensors" to "instruments" in some files (case 2 below),
    saved the rest for another PR.
• Add more explicit instrument sets in some places
  • Ocean Color CCI: {"SeaWiFS", "MERIS", "MODIS", "VIIRS"} instead of "merged". Source: https://docs.pml.space/share/s/2RVhiuK2SWyhbSthqDDoxg
  • geocat: "IMAGER (GOES 12-15)" instead of "goes_imager". The source attribute is just "goes", so there's no way
    to distinguish between 8-11 and 12-15.

Why not change all reader YAMLs? Because this would add too much complexity to this PR. Instrument names in readers/*.yaml serves two purposes, again.

1. Identify applicable composites/enhancements.
2. Add them to dataset attributes in the file handler (only some actually do this)

Translating them to WMO names is not always straightforward, so I only changed to WMO names in case 2 and saved
case 1 for a future PR. In particular:

• Specifying dependencies via instrument_name: visir/<sensor> doesn't work anymore,
  because WMO names might contain slashes and parentheses ("AVHRR/2"). Needs a new "dependencies" attribute.
• Some enhancement/composite YAMLs are not compatible with the normalization scheme and need to be renamed (e.g. mersi-1.yaml -> mersi1.yaml)
• In some cases there are multiple WMO names for basically the same instrument ("MWR (Sterna)", "MWR (AWS)" etc),
  but they all use the same composites/enhancements. So instead of copying the composites/enhancements we
  decided to stick to just "MWR". But that's not a valid WMO name.
• Sometimes the instruments attribute is not even an instrument (e.g. scatterometer, hsaf). The sole purpose
  of that attribute is to link composites/enhancements.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[sfinkens]

Sorry, wrong target branch, correct one: sfinkens#3