Improve OpenMP configuration (!109)

## What is the bug
`OpenMP_C` is not used and cannot be found with Mac clang, which causes compile trouble. This MR solves #25

.
## How do you fix it
Only find OpenMP and the link for Fortran.
Along the way, one bug for `insert_dimension` is also fixed with this MR.


Approved-by: Pradipta Samanta <samanta@dkrz.de>
Merged-by: Pradipta Samanta <samanta@dkrz.de>
Changelog: bugfix