Draft: C++ port of horizontal/mo_lib_gradients.F90

Port the functions of mo_lib_gradients.F90 to C++, with C bindings. Untested, but compiles.

cmake/get_library.cmake

@@ -53,6 +53,9 @@ function(get_library package_target tag)
   if(NOT TARGET ${target})
     find_package(${package} CONFIG QUIET)
     if(NOT ${package}_FOUND)
+      if("${CMAKE_VERSION}" VERSION_GREATER_EQUAL "3.24")
+        cmake_policy(SET CMP0135 NEW)
+      endif()
       include(FetchContent)
 
       if(NOT ARG_QUIET)
@@ -61,7 +64,7 @@ function(get_library package_target tag)
 
       FetchContent_Declare(
         lib${package}
-        GIT_REPOSITORY git@gitlab.dkrz.de:icon-libraries/lib${package}.git
+        GIT_REPOSITORY https://gitlab.dkrz.de/icon-libraries/lib${package}.git
         GIT_TAG ${tag})
 
       set(save_BUILD_TESTING "${BUILD_TESTING}")