Add OpenACC collapse clause to a nested loop (!105)

## 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.

Approved-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: feature