Use new (misc) keys in MTG2 actions

[tweska]

New prefixed (with misc-) keys should be used in scale, statistics-mtg2 and encode-mtg2 actions.

If you review this PR, I suggest going commit by commit.

The main changes in this PR:

• All non-mars keys are prefixed with misc- in their definition
• The MiscRecord can be dumped without this misc- prefix, this is needed in the encode-mtg2 action
• Removed unused code in the statistics-mtg2 action
• Created records for the metadata in the statistics-mtg2 action to enable the proper parsing/defaulting upon reading

Some improvements that we should discuss + my opinion:

• There are still direct accesses to the metadata in the statistics-mtg2 action, this should be refactored further to access everything through the record
• We parse the record (and create the config) several times in statistics-mtg2, it should be parsed once and then passed around
• FullMarsRecord and MiscRecord are defined outside the encode-mtg2 action, but to my knowledge only used there, we should define the records in that action header instead
• The FlushKind enum is not really a statistics-mtg2 specific thing, it should be moved
• May need to discuss serverRank and restartDateTime keys
• MarsMiscGeo.h now contains some random keys which are not mars, nor grib, perhaps we should create a new file MultioKeys.h?
• We should check misc-bitmapPresent before reading misc-missingValue Implemented for scale and average-rate actions
• misc-bitmapPresent and misc-missingValue don't have default values, this is especially confusing for the bitmapPresent. It can now be optional, and it is unclear if that means the bitmap is present or not. I should either be a required key, or defaulted to true/false. Defaulted bitmapPresent to false

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-251

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.70370% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.32%. Comparing base (210c956) to head (4b03c9a).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/multio/action/statistics-mtg2/MetadataKeys.cc	27.02%	27 Missing ⚠️
src/multio/datamod/core/EntryDumper.h	0.00%	13 Missing ⚠️
...c/multio/action/statistics-mtg2/OperationWindow.cc	40.00%	12 Missing ⚠️
src/multio/action/statistics-mtg2/Statistics.cc	63.63%	8 Missing ⚠️
...ion/statistics-mtg2/cfg/StatisticsConfiguration.cc	87.80%	5 Missing ⚠️
src/multio/action/average-rate/AverageRate.h	57.14%	3 Missing ⚠️
src/multio/action/encode-mtg2/EncodeMtg2.cc	0.00%	3 Missing ⚠️
src/multio/action/scale/Scale.h	62.50%	3 Missing ⚠️
...statistics-mtg2/mappings/StatisticsParamMapping.cc	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #251      +/-   ##
===========================================
+ Coverage    56.30%   56.32%   +0.01%     
===========================================
  Files          321      323       +2     
  Lines        21177    21118      -59     
  Branches      1684     1686       +2     
===========================================
- Hits         11924    11894      -30     
+ Misses        9253     9224      -29     

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