Convergence plot optimisation

[samtygier-stfc]

Some optimsations for the the convergence plot in the reconstruction progress window

Desired Behaviour

• Only create the PlotWidget the first time set_progress_plot() is called
• Only call update_progress_plot() when there is something new to plot (the loss is only calculated every 10th iteration). (maybe if the last element in iterations is the same as the current step, or something like that).

Current Behaviour

PlotWidget is always created, even when it is not used

Plot gets updates on every iteration, even if nothing has changed

#2399 (review)