MTG2 developments for SEAS6 and conversions

[tweska]

📋 Metkit Documentation 📋
https://sites.ecmwf.int/docs/metkit/pull-requests/PR-251

[codecov-commenter]

Codecov Report

❌ Patch coverage is 21.51703% with 2535 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.66%. Comparing base (df167e0) to head (a889e8f).

Files with missing lines	Patch %	Lines
...ime-spec/shapes/impl/IFSFakeDoubleLoopSingleLoop.h	11.37%	187 Missing ⚠️
...metkit/mars2grib/api/Mars2GribTestCaseGenerator.cc	0.00%	159 Missing ⚠️
...me-spec/shapes/impl/AIFSFakeDoubleLoopSingleLoop.h	12.64%	152 Missing ⚠️
...uct-time-spec/shapes/impl/AIFSStandardSingleLoop.h	14.64%	134 Missing ⚠️
src/metkit/mars2grib/CoreOperations.h	0.75%	132 Missing ⚠️
...c/metkit/mars2grib/api/Mars2GribApiErrorHandling.h	1.63%	120 Missing ⚠️
...e-spec/shapes/impl/AIFSFromStartSingleLoopAtZero.h	17.32%	105 Missing ⚠️
...spec/shapes/impl/AIFSFromStartSingleLoopPositive.h	16.26%	103 Missing ⚠️
...dels/product-time-spec/detail/ShapeNormalization.h	37.32%	89 Missing ⚠️
src/metkit/mars2grib/api/Mars2Grib.cc	7.77%	83 Missing ⚠️
... and 68 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #251      +/-   ##
===========================================
- Coverage    50.95%   43.66%   -7.29%     
===========================================
  Files          384      448      +64     
  Lines        15467    20187    +4720     
  Branches      1310     1440     +130     
===========================================
+ Hits          7881     8815     +934     
- Misses        7586    11372    +3786     

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