Implement C++ code for divrot

What is the new feature

Implement C++ functions for the Fortran mo_divrot module.

How is it implemented

This first version is implemented by simple templated functions with Kokkos.

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!

src/horizontal/CMakeLists.txt

@@ -11,6 +11,7 @@
 
 add_library(
   iconmath-horizontal
+  mo_lib_divrot.cpp
   mo_lib_divrot.F90
   mo_lib_laplace.F90
   mo_lib_gradients.F90)
@@ -57,7 +58,7 @@ target_include_directories(
     # Path to the internal C/C++ headers (for testing): Requires CMake 3.15+ for
     # multiple compile languages
     # https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html
-    $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C,CXX>:${CMAKE_CURRENT_SOURCE_DIR}>>
+    $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C,CXX>:${PROJECT_SOURCE_DIR}/src>>
   PRIVATE
     # Path to config.h (for C and C++ only): Requires CMake 3.15+ for multiple
     # compile languages