Remove unneeded CMake modules (Minor cleanup)

* Removed `BornAgain/PythonAPI.cmake`:  
  The Python API is produced via `BornAgain/multipython/MakePythonAPI.cmake`.
  - Moved the SWIG-related contents of `PythonAPI` to `multipython/ConfigureSwig`.
  - Moved the Python-related contents of `PythonAPI` to `multipython/MakePythonAPI`.
* Removed `SwigLib.cmake`:  
  The SWIG-produced API is made using `BornAgain/multipython/MakeSwigLib.cmake`.
* Removed `MakeLib.cmake`:  
  The shared library is made using `BornAgain/multipython/MakeSharedLib.cmake`.
* Moved `PythonDocs` module to `BornAgain/multipython` folder so that all Python-related CMake modules are gathered under `BornAgain/multipython` folder.
* CMakeLists.txt<root>: Minor cleanup and improvements.

See merge request !601

The SWIG-produced API is made using `BornAgain/multipython/MakeSwigLib.cmake`.

The shared library is made using `BornAgain/multipython/MakeSharedLib.cmake`.

All Python-related CMake modules are gathered under `BornAgain/multipython` folder.

The Python API is produced via `BornAgain/multipython/MakePythonAPI.cmake`.

The SWIG runtime is produced in `multipython/ConfigureSwig.cmake`.

Also, slightly improved `ConfigureSwig.cmake`.

The unit and functional tests, as well as the example tests, can be built for _all_ Python versions; i.e., the main version and the Python versions set via `BORNAGAIN_PYTHON_PACKAGE_VERSIONS` variable.

The tests for the extra Python versions are built by setting the variable `BORNAGAIN_PYTHON_PACKAGE_TESTS=ON`.

* A `tests` subfolder is added to the main binary directory to to store the built tests for the _main_ version and their output.

* A `tests` subfolder is added to the build directory for Python packages to store the built tests and their output.

* Adapted unit and functional tests for multiple Python versions.  
  __NOTE:__
  - `Functional/PyEmbedded` test still cannot be performed for the Python packages, most probably due to a problem with proper usage of the Python C/API. Currently, it is produced only for the main version.
  - The GUI test is performed only for the main version, since the Python packages does not include a GUI.

* A new CMake module `MakePythonAPI` is introduced to produce the Python interfaces to the C++ libraries. `MakePythonAPI` includes the necessary CMake code to build the Python interfaces and Python packages (via `MakeSwigLib` and `MakePythonWheel` modules). In this way, the Python-related part of the build mechanism is enclosed in `MakePythonAPI`.

* Differentiated between `BORNAGAIN_PYTHON` variable and `WITH_PYTHON_API`
  argument:
  * `BORNAGAIN_PYTHON` = Add Python-related functionality to the C++ libraries.
  * `WITH_PYTHON_API` = Add a Python interface to the C++ libraries; currently made with SWIG.

* Adapted `CMakeLists.txt` for all modules (`Base`, `Fit`, etc.) to the new interface of `MakeSharedLib`.

* GUI:
  * `SampleListView`: Added proper preprocessor guards for `BORNAGAIN_PYTHON`.
  * Improved and cleaned up `CMakeLists.txt`.
  * Test: Minor improvements.

See merge request !593

Also, explicitly add the required object files as source to the libraries.

* The Python interface to a shared library is made via MakePythonAPI module.
  - Moved installation-related parts from MakeSwigLib to MakePythonAPI;
  - Moved Wheel-related parts from MakeSwigLib to MakePythonAPI.

* Differentiate between `BORNAGAIN_PYTHON` variable and `WITH_PYTHON_API`
argument:
  + BORNAGAIN_PYTHON = Add Python-related functionality to the C++ libraries;
  + WITH_PYTHON_API = Add a Python interface to the C++ libraries;
    currently made with SWIG.

* Add OUTPUT_DIR to MakeSharedLib to allow setting an output folder for the shared library.

Furthermore, make the test output directory only when required

fix mem leakage and discussions of MR !569

See merge request !574

Sample item serialization

See merge request !573