quantity.Discrete without grid

Maybe its more practical, if there are no quantity.Discrete without a grid defined. If a mathematical operations removes all grid, for instance subs() or int(), then a double array could als be retourned. Since all mathematical operations of quantity.Discrete should be compatible with double-arrays, this should work easily. Additionally, this might enease working with quantity.Discrete, since special cases with empty grids do not need to be considered anymore. Additionally, the isempty(quantity.Discrete())-problem could be fixed easily by checking isempty(grid) additionally.