Idea: Test do concurrent

[tkoskela]

Take one of the OpenMP parallel loops from #195 and rewrite it using do concurrent. Compare performance.

https://www.intel.com/content/www/us/en/docs/fortran-compiler/developer-guide-reference/2023-1/do-concurrent.html https://developer.nvidia.com/blog/accelerating-fortran-do-concurrent-with-gpus-and-the-nvidia-hpc-sdk/