Merging branch 'j.20'  into 'r21'.

See merge request !2541

corrections:
- restore var/mk_pypack_linux.sh call
- suppress warning from clang about Root

Merging branch 'r21.1'  into 'r21'.

See merge request !2047

Merging branch 'a.fixR21.1'  into 'r21.1'.

See merge request !2029

Merging branch 'fixMacPackage'  into 'r21.1'.

The major change to the packaging script is meant to repair and improve the packaging mechanism for MacOS.
The MacOS package was broken due to the new dependencies of the Qt libraries and the migration to [pyenv](https://github.com/pyenv/pyenv) platform.

The packaging script, `devtools/deploy/mac/mac_package.py`, is fully re-written in Python for clarity, as shell scripting was not suitable for such a complicated task. In essence, the script follows the rules explained in `man dyld (1)`. Furthermore, it uses MacOS native tools `otool` and `install_name_tool` to adjust the binaries.

The script finds the dependencies of the core libraries up to a maximum level. The dependencies are then copied to the corresponding package directories, and library references and RPATHs are modified accordingly.

The Python platform for building will be [pyenv](https://github.com/pyenv/pyenv) (on Windows: [pyenv-win](https://github.com/pyenv-win/pyenv-win)).

See merge request !1988

<root>/CMakeLists.txt: add 'BornAgain_LIBRARIES' cached variable to store paths to BornAgain core libraries