Add std::hash specializations for Date, Time, DateTime

[Ozaq]

Description

Allow Date, Time, and DateTime to be used as keys in std::unordered_map and std::unordered_set by providing std::hash specializations in each type's header.

Date hashes via julian(), Time via hhmmss() (avoiding floating-point hash pitfalls on the underlying double), and DateTime combines both using a new hash_combine utility.

Requires ecbuild fix for sg.pl to handle template <> syntax.

Fixes: ECKIT-561

NOTE: blocked by a bug in ecbuild/sg.pl ecmwf/ecbuild#134

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.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/eckit/pull-requests/PR-278

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.41%. Comparing base (9daf037) to head (c0f85c9).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #278      +/-   ##
===========================================
+ Coverage    66.38%   66.41%   +0.02%     
===========================================
  Files         1127     1128       +1     
  Lines        57792    57829      +37     
  Branches      4408     4408              
===========================================
+ Hits         38366    38406      +40     
+ Misses       19426    19423       -3     

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[wdeconinck]

ecmwf/ecbuild#134 has been merged and up for next release.
Why is this blocked by ecbuild?
Does this mean the ecbuild required version needs to be bumped? (it would be better if not)

[Ozaq]

ecmwf/ecbuild#134 has been merged and up for next release. Why is this blocked by ecbuild? Does this mean the ecbuild required version needs to be bumped? (it would be better if not)

The ecbuild version number needs to be bumped and eckit needs to update min version. Without the modifications to sg.pl this change won't be able to pe processed by ecbuild.

In the CI this would work (always uses develop) but break on dev machines / ATOS / elsewhere unexpectedly