Added the option im_enable_openmp to compile the library with the openmp flag

What is the new feature

The library now compiles OpenMP codes even when the OpenMP flag is not added manually in CMAKE_Fortran_FLAGS

How is it implemented

Added the option IM_ENABLE_OPENMP, checked whether OpenMP is available as a package and added the flags in the existing Fortran_FLAGS

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions
• Prior to merging, please remove any boilerplate from the MR description, retaining only the Please describe your feature in a couple of words and describe important implementation details of the feature section to maintain

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