Merging branch 'j.0'  into 'main'.

See merge request !2739

[a.cleanUpSWIG_nodeChildren] Hide vector of pointers and related methods from the SWIG Python API (#127)

Merging branch 'a.cleanUpSWIG_nodeChildren'  into 'main'.

The methods which produce a `std::vector<const INode*>`, `std::vector<const Layer*>` and `std::vector<const IReParticle*>` are hidden from the SWIG-produced Python API.

These methods are not needed for the Python interface.

Related to issue #127

See merge request !2738

hide std::vector<const Layer*> unwrapped

Merging branch 'j.6'  into 'main'.

See merge request !2737

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

See merge request !2736

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