cpp version of mo_lib_intp_rbf::rbf_vec_interpol_vertex_lib (!34)

## 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.

Co-authored-by: Pradipta Samanta <samanta@dkrz.de>
Merged-by: Pradipta Samanta <samanta@dkrz.de>
Changelog: feature