Fix to grid_sample, avoiding bugs which discarded duplicate samples

grid_sample on SheetStack would previously generate a bounds and
density depending on the supplied rows, cols and lbrt. This caused
some samples to be discarded or snap to the wrong coordinates.
This fix retains the original densities and merely adjusts the
bounds if an lbrt is supplied.
Fixed GridPlot to place subplots in correct global axis position
Added style to GridLayout
Added common_keys and type methods to Grid and made minor fixes
Enabled passing plotopts to Grid and GridLayout