mars2grib: remove __builtin_unreachable()

[tweska]

Remove all occurrences of mars2gribUnreachable();, which is an alias for __builtin_unreachable(). I found only two cases where removing the __builtin_unreachable() actually produces a warning and these cases can be rewritten to avoid it.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.88%. Comparing base (5056555) to head (21cefcf).

Files with missing lines	Patch %	Lines
.../mars2grib/backend/deductions/numberOfTimeRanges.h	0.00%	5 Missing ⚠️
...ars2grib/backend/deductions/statisticsDescriptor.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #173      +/-   ##
===========================================
- Coverage    60.89%   60.88%   -0.01%     
===========================================
  Files          309      309              
  Lines        11732    11753      +21     
  Branches      1035     1035              
===========================================
+ Hits          7144     7156      +12     
- Misses        4588     4597       +9     

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[tweska]

This is the only case where I have not been able to remove the __builtin_unreachable() for the Intel compiler. It is not possible to collapse the else branch into the parent scope here.