Re #8637. Clone the input workspace if filtering or masking.

Avoids the input workspace potentially being changed by the algorithm.
Re #8637. Lose the flag for reloading the data.

No longer needed now that I'm good and never modify the input workspace
to the StepScan algorithm.
Re #8637. Add capability to specify multiple runs.

Each run is treated as a single point in the scan (in the order they're
given), a scan_index entry is inserted and the runs are merged together.
They can then be treated as other scan runs. This allows scans from
non-ADARA-enabled instruments to use the functionality of this interface.

The loading algorithm had to be changed from LoadEventNexus to Load for
this. This then required changes around that code to catch if the user
tries to load the wrong kind of file.
Re #8637. Disable GUI and show hourglass while merging runs.
Re #8637. Add a scan_index=0 entry at the end time of each run.

Will make the normalization and calculations of average log values more
correct.
Re #8637. Add a text label that can be used to report status.

And add reporting of what operation the interface is undertaking where
it might take a noticeable amount of time.
Re #8637. Make the GUI more responsive...

...by running algorithms asynchronously and processing events in the
meantime so that the whole of MantidPlot doesn't lock up.
Merge branch 'feature/8637_stepscan_preadara_group' into develop