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

src/mo_fortran_tools.F90

@@ -1813,7 +1813,7 @@ CONTAINS
 
     minval_2d = HUGE(minval_2d)
 
-    !$ACC PARALLEL LOOP GANG VECTOR DEFAULT(PRESENT) ASYNC(1) REDUCTION(MIN: minval_2d) IF(lzacc) COPY(minval_2d)
+    !$ACC PARALLEL LOOP GANG VECTOR DEFAULT(PRESENT) ASYNC(1) COLLAPSE(2) REDUCTION(MIN: minval_2d) IF(lzacc) COPY(minval_2d)
     DO j = 1, s2
       DO i = 1, s1
         minval_2d = MIN(minval_2d, var(i, j)) ! The loop is equivalent to MINVAL(var(:,:))