libBornAgainSim.i: use 'PyObject_CallFunctionObjArgs' instead of 'PyObject_CallOneArg' for compatibility with ealier Python versions

Merging branch 'a.cleanUpSWIG_customFF'  into 'main'.

The SWIG layer to `Sample` module is extended with a functionality to wrap a custom Python Formfactor object.

The test for custom formfactor (`scatter2d/CustomFormFactor.py`) is restored.

The Python test script is improved with explanatory comments.

See merge request !2756

Merging branch 'm.1'  into 'main'.

See merge request !2758

Merging branch 'm.0'  into 'main'.

See merge request !2757

[a.cleanUpSWIG_ImportSample] Improve the construction of a 'Sample' from a Python script (Closes #1028)

Merging branch 'a.cleanUpSWIG_ImportSample'  into 'main'.

Based on the recent changes in the SWIG layer, the task of obtaining a `Sample` from a Python script is divided into two parts:
* Reading and running the Python script, performed in `PyCore/Embed/PyInterpreter` module.
* Constructing a C++ `Sample` instance from a Python `Sample` instance, performed in the SWIG layer `libBornAgainSample.i`.

This redesign leads to some simplifications and improvements in the BornAgain structure:
* The module `PyCore/Sample/ImportSample` is not needed.
* The extra SWIG header file `swig_runtime.h` is not needed.
* The extraction of the C++ `Sample` instance from the Python object is performed in the `Sample` module (not in the `PyCore` module).

The module `PyCore/Embed/PyInterpreter` is reviewed and improved.
The initialization and finalization of the embedded Python interpreter is performed properly.

Tests are done with Python scripts under Linux.

Resolves issue #1028

See merge request !2755

Fixes issue #1028

Note that the check for Python initialization is moved to
'PyCore/Embed/PyInterpreter' module.

libBornAgainSample.i: define BA_SWIG_sampleFromPyObject to extract a C++ Sample instance from a Python object

Merging branch 'm.0'  into 'main'.

See merge request !2754

Merging branch 'm.0'  into 'main'.

See merge request !2752