cpp version of mo_lib_intp_rbf::rbf_vec_interpol_vertex_lib

What is the new feature

cpp version of mo_lib_intp_rbf::rbf_vec_interpol_vertex_lib

How is it implemented

Kept separate file from other mo_lib_intp_rbf routines to avoid merge conflicts. Will be resolved by Ali and Dylan later.

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
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)
• 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

You are not supposed to merge this request by yourself, the maintainers of libiconmath take care of this action!