kalman smoother deep copy - unpickable, loose reference

[ppgp-gmv]

Stone Soup v1.4, smoother/kalman.py line 202
smoothed_track = copy.deepcopy(track, {id(track.states): smoothed_states})
Creates a deep copy, with two issues:

• Cannot deal with unpickable objects (e.g. metadata contains unpickable objects used in Detection class to keep track of detections and the observations that generated the detection for subsequent processing...)
• Looses "pass by reference" functionality.

The first can be worked around, the second aswell but seems a bit more inconvenient and prone to raise errors in postprocessing activities for instance

[sdhiscocks]

Thanks for the report @ppgp-gmv. Yeah, a shallow copy is probably preferable, and that would solve the issues you've raised?

[ppgp-gmv]

Yes, I was discussing this issue with Lyudmil, and proposed that same approach (which I am going to implement for now overloading)