Hide 'clone' method from the SWIG Python API

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 (closed)