Retrieves a handle to a previously-open SliceViewerWindow.
This allows you to get a handle on, e.g., a SliceViewer that was open
by the MultiSlice view in VATES Simple Interface.
Will raise an exception if not found.
Args:
source :: name of the workspace that was open
label :: additional label string that was used to identify the window.
Returns:
a handle to the SliceViewerWindow object that was created before.
