Enable Step Scan interface to accept series of non-ADARA runs as points in the scan

[mantid-builder]

Original Reporter: Russell Taylor

This ticket is blocked by :

• TRAC8638
• TRAC8669

Pre-ADARA, a scan could be saved as a series of runs where each run represents a point in the scan (in order). We should be able to construct a workspace that will work with StepScan by adding the scan_index log with the appropriate value for each workspace and then adding the workspaces together.

[mantid-builder]

This issue was originally trac ticket 8637