Fix missing coordinate variable attributes

[pawel-wolff]

Description

Since v1.0rc a regression bug was introduced: attributes of some coordinate variables, notably "forecast_reference_time" and "valid_time", were gone.

This PR fixes the bug, provides relevant tests and adds standard_name and long_name attributes for the coordinate variable member.

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 81.51%. Comparing base (df0cc0b) to head (df964b3).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1001      +/-   ##
===========================================
+ Coverage    81.48%   81.51%   +0.03%     
===========================================
  Files          236      236              
  Lines        15444    15472      +28     
  Branches       754      756       +2     
===========================================
+ Hits         12584    12612      +28     
  Misses        2634     2634              
  Partials       226      226              

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