Refs #7451. DGSReduction parallel button in GUI
Allow multiple file load in DGSReduction gui. Refs #7451
Expose correct method to python. Refs #7451

Updated FlatBackground call to CalculateFlatBackground. Needs to be
tested for events mode.
Small changes to DgsReduction to allow multiple files. Refs #7451
Additions to ui. Refs #7451
Updates to DGS gui. Refs #7451
Fix batch for adding files. Refs #7451
Merge remote branch 'origin/feature/7451_ParallelDGS'