Added dimension_keys method to NdMapping to returns keys and labels
Extended NdMapping contains method to find tuple and non tuple keys
Changed SheetOverlay ROI behavior

When overlaying SheetView objects the created SheetOverlay object
will inherit the ROI from the component SheetView. If both component
SheetViews have an roi_bounds, it will be inherited from the
SheetView left of the multiply operator. When calling ROI on the
SheetLayer it applies it's inherited ROI to all the layers within
it. This way layers in a SheetOverlay can retain independent
roi_bounds and a ProjectionGrid can overlay the same SheetView
object on all elements in the grid.
Extended SheetStack mul operator

SheetStack mul operator now allows combining SheetStacks with
different dimension_labels as long as one is a subset of the other.