ECC-1734: GRIB2: typeOfTimeIncrement=1 removes endStep and stepRange

[mjg41]

Description

Setting typeOfTimeIncrement=1 erroneously set "stepType=avgd" (even when the period is e.g. 6h) and removes the computing of endStep and stepRange which erroneously become zero.

avgd has the undesirable behaviour of removing the endStep and stepRange, which is incorrect.
We contain this undesirable behaviour to pre-mtg2 data only to keep backwards compatibility but have post-mtg2 interpreted correctly.

This passes all ctests and regression tests.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.53%. Comparing base (93b19e5) to head (3627369).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #447   +/-   ##
========================================
  Coverage    88.53%   88.53%           
========================================
  Files          849      849           
  Lines        63097    63101    +4     
  Branches     11236    11238    +2     
========================================
+ Hits         55862    55866    +4     
  Misses        7235     7235           

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