Sort CMake install files consistently in `CMakeLists.txt` files

[jhlegarreta]

The file list saved to the FILES variable to be provided to the CMake install command should follow the same sorting convention to improve the consistency and readability of the CMakeLists.txxt files.

Currently we find both instances:

install(FILES Code.cxx CMakeLists.txt Code.py

and

install(FILES Code.cxx Code.py CMakeLists.txt

It probably makes sense to group together or list consecutively all source code files (second instance) and have CMakeLists.txt either at the beginning (install(FILES CMakeLists.txt Code.cxx Code.py) or at the end (second instance). Although the former is not used, since CMakeLists.txt will always be present, it is "easier" to append a filename rather than inserting it.

This is an easy change to do (although it requires manual work unless some clever script to automate the process is developed) provided that we reach consensus or establish a criterion.