Fixes to Table and TableStack dframe methods
Moved map method from Stack to NdIndexableMapping level
Fixes to SheetStack grid sampling method
Fixes to handling of CoordinateGrids in ViewOperation

ViewOperation now checks whether the type of View returned by a
ViewOperation is a SheetLayer to determine whether to use a
CoordinateGrid or DataGrid. Additionally xdensity and ydensity
are passed directly rather than relying on fragile calculation
computing densities from bounds and shape.