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

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

See merge request !2753

Merging branch 'a.cleanUpSWIG_Fit'  into 'main'.

The SWIG API to the `Fit` module is revised completely to make the minimizer's call to Python functions and conversion of results more explicit.
Auxiliary functions are added to call a Python callable with a `Parameters` instance and convert the result to `double` or a C++ `std::vector` as required.

The Python extension to the `Minimizer` class is updated and improved.

The unneeded `PyCallback` C++ class and `ParametersIterator` Python class are removed.


See merge request !2747

The method 'MinimizerResult Minimizer::minimize(const PyCallback&,
const Parameters&)' is not needed.