Support for single precision wp

What is the new feature

Support for single-precision as working-precision in libfortran-support. Related to icon-mpim MR (https://gitlab.dkrz.de/icon/icon-mpim/-/merge_requests/558), and libiconmath MR (libiconmath!13 (merged)).

How is it implemented

• New CMake argument FS_ENABLE_SINGLE_PRECISION, disabled by default
• Precision handling: Introduction of src/mo_iconlib_kind.f90 shared by fortran-support and iconmath:
  • Precision (dp+sp): Using iso_fortran_env variables dp=>real64 and sp=>real32
  • Working precision (wp): FS_ENABLE_SINGLE_PRECISION==TRUE -> wp=real32, else wp=real64
  • Variable precision (vp): FS_ENABLE_MIXED_PRECISION==TRUE -> vp=real32, else vp=wp
• Extensions of some functions to include sp and dp interfaces
• Kind for t_ptr types is wp by default, with wp or dp or sp versions specified by suffix.

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

• Test coverage does not decrease
• 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
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

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