3D ROI fixed and ImageJ parsing

The ImageJ ROI parser will now also grab the Z-plane.
Updated the import_ImageJ_rois example script.
Couple of fixes for 3D ROIs in the ROI class.

Note: previously if any plane had an empty ROI mask, an exception would be thrown. It's now fixed to never throw an exception, but it might return an empty MultiPolygon object. What should we do if you pass in a completely empty ROI? I think it's OK as is, but we could have it raise an exception if all planes are empty.