Improve support for gridSpec in encoder

[tweska]

All features in support of mode when skipSection3 is enabled + fixes for a few issues

• Allow supplying gridSpec json strings as well in grid
• Re-enable prepping the sample for spherical harmonics
  • Otherwise, setting gridSpec at the end won't work for SH
• Read gridSpec from the sample to set MARS grid in grib2mars
  • Key truncation is no longer set!
• Sets ECCODES_ECKIT_GEO=1 in the environment if skipSection3 is enabled in any of grib2mars, mars2mars or mars2grib
• Add roundtrip tests by Pedro
• Enable skipSection3 in grib-to-mtg2 tool
• Misc dictionary can be merged behind mars2mars API (no need to duplicate the code everywhere)
• Error out when scaling/offsetting values with spherical harmonics (this is not supported in eccodes)

CI fails because it relies on the truncation method being available on the Grid class in eckit::geo. This is not in develop yet, it seems.

The following packages have branches that work with this branch/PR in metkit:

• ecKit: feature/eckit-geo
• MIR: fix/metkit-encoder-testFix metkit unit tests mir#55
• MultIO: fix/mars2mars-api-call Remove misc overwrite logic multio#295

This PR contains the fix from #259

📋 Metkit Documentation 📋
https://sites.ecmwf.int/docs/metkit/pull-requests/PR-257

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.62948% with 144 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (50r2-dev@3693c2d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
tests/grib2grib/roundtrip.cc	43.36%	64 Missing ⚠️
src/metkit/mars2mars/api/Mars2Mars.cc	14.03%	49 Missing ⚠️
src/metkit/grib2mars/api/Grib2Mars.cc	43.75%	9 Missing ⚠️
src/metkit/mars2grib/backend/encodeValues.h	20.00%	8 Missing ⚠️
src/metkit/grib2mars/mappings/rules/truncation.h	72.72%	6 Missing ⚠️
src/metkit/mars2grib/api/Mars2Grib.cc	62.50%	6 Missing ⚠️
src/metkit/grib2mars/mappings/rules/grid.h	80.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             50r2-dev     #257   +/-   ##
===========================================
  Coverage            ?   44.47%           
===========================================
  Files               ?      465           
  Lines               ?    21064           
  Branches            ?     1521           
===========================================
  Hits                ?     9369           
  Misses              ?    11695           
  Partials            ?        0           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.