[Task] Solve the issue with unit changing in Rietveld tab

1. Load in some sample data (as attached here, NOM161121tof.gsa, NOM161122tof.gsa, NOM161123tof.gsa) in Rietveld tab and select nothing in Bragg Workspaces tree and click on any of the item in the unit change dropdown menu will crash ADDIE.

   • According to the error message, we need to go to the switch_bragg_unit function definition in addie/rietveld/event_handler.py and search for the line containing At least 1 GSS file must be selected.. We want to replace such an assertion operation with the following codes to throw out warning message instead of crashing ADDIE in such a situation,

     if len(ws_group_list) == 0:
         print("[Warning] At least 1 GSS file must be selected.")
         return

2. Load in some sample data (as attached here, NOM161121tof.gsa, NOM161122tof.gsa, NOM161123tof.gsa) in Rietveld tab and select workspaces in Bragg Workspaces tree and click on any of the item in the unit change dropdown menu will crash ADDIE.

   • According to the error message, we need to go to the switch_bragg_unit function definition in addie/rietveld/event_handler.py and search for the line containing if 'workspaces' in ws_group_list. We see that workspaces is intended to be removed from workspaces list but we have a typo here - workspace needs to be changed to workspaces.