Merge the outer `gcmpy` `CMakeLists.txt` file with the `GEOSgcm_App` `CMakeLists.txt` file.

[JulesKouatchou]

The gcmpy package has an outer CMakeLists.txt file. When the tool will be ready, we need to make sure that the CMakeLists.txt file does not overwrite the one in the folder GEOSgcm_App. We need to create a mechanism to merge the two CMakeLists.txt files.

[JulesKouatchou]

It is important to note that this issue will also involve modifying all the internal gcmpy CMakeLists.txt files (to edit the location where files are installed) and the pathdict settings in the file gcmpy/setup_tools/setup_envs.py.