[makePythonWheel_I_i223] Make Python packages ('wheels') for multiple platforms (#223/1)

Merging branch 'makePythonWheel_I_i223'  into 'main'.

The following changes carry out the _first_ step described in issue #223.

The CI mechanism is modified to make a Python package for Linux, MacOS, and MS-Windows.
The changes are kept minimal, as far as possible. The new CMake modules are placed in `cmake/multipython` folder.

* In the root `CMakeLists.txt`,  variables `BA_PY_PACKAGE` and `BA_PY_PLATFORM` are added to build a Python package.
  - `BA_PY_PACKAGE = ON` builds a Python package with the default Python platform, or a given Python platform (via `BA_PY_PLATFORM`).
  - If `BA_PY_PLATFORM = <path/to/Python/platform>` is provided, CMake tries to find a Python platform in the given path via configuration files, to build the Python package.
    If `BA_PY_PLATFORM` is given, there is no need to set `BA_PY_PACKAGE = ON`.
  + Example:
    ```
    cmake .. BA_PY_PACKAGE=ON
    cmake .. BA_PY_PLATFORM=/path/to/python38
    ```

* `multipython/FindCustomPython3` searches for a Python platform in the default paths or in a given path.

* `multipython/PyDirectories` makes the required directories for the Python package under the build directory.

* `multipython/PyDependences` finds the required Python dependencies and performs the required configuration.

* `multipython/MakePythonWheel` makes the Python wheel.

* Added proper RPATHs as hints to load the dependencies of the libraries at run-time (on Linux & MacOS).
  On MacOS, a shell script, `devtools/deploy/mk_pypack_macos.sh`, is used to gather the required external dependencies, correct the library ids and RPATHs and make the Python wheel. A similar script, `devtools/deploy/mk_pypack_linux.sh`, is used to make the Python wheel on Linux.

* The files and directory structure which Python needs to build the 'wheel' are stored under `Wrap/PythonPackage`.

* Removed the unnecessary dependence on Boost::regex which is added automatically by CMake `FindBoost` module.

* The GitLab CI script is updated to build the Python package and keep it as an artifact.


Related to issue #223 and milestone %5

See merge request !766