Merging branch 'a.cleanUpSWIG'  into 'main'.

The `clone` method is removed from the SWIG Python API, as it relies on the ownership transfer mechanism in C++ which is absent (or hard to reproduce in Python) and might lead to hidden memory leaks.

The public interfaces of `Datafield` and `Frame` are modified so that no implicit ownership transfer is performed.
The rest of the code is adapted accordingly to use the new interface.

Due to this change, a single test, `Example.scatter2d.CustomFormFactor.persist` does not pass. Passing this test needs more changes which are left for the next merge request.

Related to #127

See merge request !2735

The 'clone' method relies on the ownership transfer mechanism in C++ and should
not be exposed to the Python interface to avoid possible memory leaks.

Merging branch 'm.i20'  into 'main'.

See merge request !2734

Merging branch 'm.i20b'  into 'main'.

See merge request !2731

Merging branch 'm.fix_rough_seed'  into 'main'.

See merge request !2732

Merging branch 'a.cleanupSWIG'  into 'main'.

This is the first phase of the SWIG interface cleanup.

The second phase will be the discarding of the `disown` mechanism (used in the Python interface to `Sim` module) and removing the `clone` methods from the SWIG interface. Both are related to the exposure of C++ ownership-transfer mechanism to Python.

Related to issue 127

See merge request !2733