Merge branch 'multipythonTests' into 'develop2'

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