Reorganize `callback` module structure

[GiovanniCanali]

Is your feature request related to a problem? Please describe.
The callback module structure should be updated following the discussion in #735. The new internal structure is as follows:

-- callback
-------- optim
--------------- switch_scheduler.py
--------------- switch_optimizer.py
--------- refinement
--------------- refinement_interface.py
--------------- r3_refinement.py
-------- processing
--------------- metric_tracker.py
--------------- pina_progress_bar.py
--------------- normalizer_data_callback.py