Implement access to subgroups on Phase/General/Compute menu #169
Description
Right now to generate subgroups one needs to go to the histogram/Unit Cells List tree item and generate cells (good place to view cells with powder data). To create new .gpx files, one goes to the Phase/General/Compute menu and uses "Make subgroup project files." Since supergroup transforms can be done w/o the Unit Cells List tree item, it would be nice to have a way to do this from one place as well.