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.

libBornAgainFit.i: add aux. functions to call a Python callable and convert the result to double or a C++ std::vector

No need for an extra ParametersIterator class.