Reorder initialization of slice widget
Reorder initialization of slice widget

It was being shown before its position was being set. Reverse the
order of those operations so the slice isn't shown at the origin when
the data has been moved and a Slice visualization is first added.

mathturtle/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz