Re #6371. Check whether the input table is empty, fail if it is.

The algorithm previously would fail, but with a cryptic error message
at the point it tried to create the output workspace. This is more
obvious.
Re #6371. Don't remove the row for scan_index=1 if it's the only one.
Re #6371. Add code to allow use with live data.
Merge branch 'feature/6371_step_scan_gui' into develop