Wumpus

Copperbox revision 882.

I've changed the bounding box type so it has a 'zero' element and thus can support monoid rather than groupoid.

I want to change the picture type slightly so it can support pictures that have some size but no content. This is to draw lines that have a parametric representation Ax + By + C = 0. To draw a line the function must supply an area for the line to be drawn in - if the line doesn't actually enter the area the function should still return a picture without content (but still with a bounding box). I don't want to make the picture type more complicated so I've shifted some of the complexity (bounding boxes of empty pictures) to the bounding box type.