Extend set_quantity to allow addition of values

[ole]

Sometimes, there is a need for adding values to a quantity - e.g. to put buildings on top of a landscape. Polygon_function is often used for defining the buildings, but there is currently no mechanism for adding them.

I suggest allowing a new boolean keyword, addition, in set_quantity so that one can say:

set_quantity('elevation', elevation_data set_quantity('elevation', building_data, addition=True)