MARS2GRIB: Make MARS number optional in numberOfForecastsInEnsemble resolver

[tweska]

MARS key 'number' is not available in ensemble means, thus we cannot always do the validation of perturbation number in this resolver.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.18%. Comparing base (b13ae4d) to head (692c3c7).

Files with missing lines	Patch %	Lines
...b/backend/deductions/numberOfForecastsInEnsemble.h	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #156   +/-   ##
========================================
  Coverage    54.18%   54.18%           
========================================
  Files          300      300           
  Lines        11564    11564           
  Branches      1025     1025           
========================================
  Hits          6266     6266           
  Misses        5298     5298           

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