SelectVolumeWidget: guard against invalid extents from user input
SelectVolumeWidget: guard against invalid extents from user input

If the user inputs a number into the spinboxes in the SelectVolumeWidget
such that the minimum is greater than the maximum for any dimension, the
other spinbox for that dimension is updated to the same value so that no
negative volume can be created.  Invalid extents were causing crashes.

mathturtle/tomviz,cryos/tomviz,thewtex/tomviz,OpenChemistry/tomviz,cjh1/tomviz,cryos/tomviz,cryos/tomviz,mathturtle/tomviz,mathturtle/tomviz,cjh1/tomviz,cjh1/tomviz,OpenChemistry/tomviz,thewtex/tomviz,thewtex/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz