Closes issue #949

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

See merge request !2763

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

See merge request !2762

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

See merge request !2761

Merging branch 'a.fixSimSWIG'  into 'main'.

See merge request !2759

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