[ENHANCEMENT] Improvements to atom trace

[MBartkowiakSTFC]

Is your feature request related to a problem? Please describe. At the moment, the atom trace calculation can be performed for one atom at a time in MDANSE_GUI.

The following improvements have been suggested:

1. Calculate trace from a subset of trajectory frames, so one can see the trace during a finite time within the whole calculation,
2. Add a method to export the trace in a file format readable to atomic visualization software. (E.g. “volumetric data” in VESTA).
3. Calculate trace for multiple atoms at the same time.

Describe the solution you'd like 1 and 2 could be solved with a single additional widget each, plus backend code. 3 should be combined with the new atom selection mechanism.

Describe alternatives you've considered N/A

Additional context N/A