Add OpenACC collapse clause to a nested loop

What is the new feature

Improve minval_2d performance.

How is it implemented

Add COLLAPSE(2) to a nested loop in minval_2d to improve performance.

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Test coverage does not decrease
• Reviewed by a maintainer
• Incorporate review suggestions
• Prior to merging, please remove any boilerplate from the MR description, retaining only the Please describe your feature in a couple of words and describe important implementation details of the feature section to maintain
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

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