RangeSelectionOverlay fails to handle alternate metadata names.

When the metadata name attribute of a RangeSelection tool is changed to something other than 'selections' the overlay does not appear to correctly handle the change. Setting the metadata_name attribute of the RangeSelectionOverlay seems to prompt the overlay tool to search for a range which contains all of the selected points. The code indicates that it treats the metadata as a selection mask rather than a range. An exception arises when attempting to find the range that contains all of the points in the mask and appears to be related to finding the runs in the selection mask.