Find OpenMP package with REQUIRED keyword (!93)

## What is the bug
A package-NOTFOUND is undetected and does not raise a problem. See #20
This MR closes #20
## How do you fix it
The OpenMP find package is made `REQUIRED`.  
The find package for OpenACC has to stay `QUIET` or else it will try to look for both `OpenACC_C` and `OpenACC_Fortran`. (LUMI does not have `OpenACC_C` at the moment.)

Approved-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Approved-by: Sergey Kosukhin <sergey.kosukhin@mpimet.mpg.de>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: other