Add 3D solid model operations

[jonathanhogg]

This PR adds support for new !slice, !intersect, !union and !difference nodes in a !canvas3d scene that cut apart models and combine them with boolean operations. There are some important notes on this:

• models must be solid, steps will be taken to combine edges and fill holes; if necessary, the nuclear option is to compute a convex hull around the model
• because edges are combined, primitive models with sharp edges will lose their definition as their vertices are combined and vertex normals averaged – the operators will attempt to snap apart these edges again for rendering so as to retain smooth, natural shading (controlled by the smooth= and minimum_area= attributes
• solid model geometric operations are expensive, but they will be aggressively cached – if the operations have to be re-executed constantly because of dynamic parameters then it is a good idea to step these parameters so that caching will do something useful

This PR re-adds a hard requirement on scipy and introduces new ones on manifold3d, mapbox_earcut, networkx, rtree and shapely.