Re #7767. Add errors to the table output of SumEventsByLogValue.
Re #7767. Tweak the performance test.

To see if it leads to 'steadier' performance.
Re #7767. Add boilerplate test initialisation code.

Nothing to do with this ticket, but realised I missed this when I added
this performance test recently.
Re #7767. Include errors in the generated plot.
Re #7767. Change title of workspace corresponding to plot.

People are going to want to fit to this data, so it doesn't make sense
to make it a hidden workspace. Also amended the name so it starts with
the same string as the table workspace.
Merge branch 'feature/7767_stepscan_add_errors' into develop