wumpus

Copperbox revision 1565.

Work on making node drawing configurable for Wumpus-Tree.

I've also added an analogue of the Reader monads local to the DrawingCtx monad, so that drawing properties like fill colour can be changed on the fly.

Note to self - I need to add cross instances for the transformers in Basic.Monads. I've added a DrawingCtxM instance for the Turtle transformer, but I need to do the other transformers and classes.