Sesans debugging

[caitwolf]

This removes the deep copying of the model that occurs during fitting and allows the plots to update with changes after fitting. This deepcopy was done by @rozyczko so he should probably check that it is fine to remove.

This should close #1998 - This was also a problem for non-sesans data but was more apparent when working in the sesans correlation space, making it seem that the plots were not updating after fitting for sesans data only.

[rozyczko]

This deepcopy goes back A LONG time, to 2018, where I was fixing an old bug in polydispersity behaviour.
This seems to have been a leftover from that exercise. The target bug does not occur anymore with the removed deepcopy so I assume it got resolved/hidden elsewhere over the years.
As such, I think removal of the deepcopy is benign and if it fixes the issue, should be merged.

[rozyczko]

For the sake of tracking the issue:
these two bugs are linked to the change from 2018 (viewable only to ESS staff)
https://jira.esss.lu.se/browse/SASVIEW-1008
https://jira.esss.lu.se/browse/SASVIEW-1013

[butlerpd]

seems to fix the problem and a quick check of my standard 1D fitting also still works and the fact that this may also have been a minor issue for 1D fitting, I think this should be merged post haste