fix time_index

[vgro]

There was an issue with the time_index as a coordinate only, we need it to be a dimension to be accessible by isel. I tried to add a 4th dimension but that caused problems, so as an intermediate solution I renamed the time dimension and create an issue.

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov-commenter]

Codecov Report

Merging #255 (c701ba3) into develop (202baad) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #255   +/-   ##
========================================
  Coverage    95.08%   95.08%           
========================================
  Files           42       42           
  Lines         1708     1708           
========================================
  Hits          1624     1624           
  Misses          84       84           

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[jacobcook1995]

LGTM!