Add optimised bounding_sphere() calculations for more primitives.

[CnlPepper]

At present, all primitives except the Sphere primitive use the default method of calculating a bounding sphere. The default is to calculate a sphere that encloses the primitives bounding box.

The following primitives should have optimised bounding spheres:

• cone (basic geometry)
• cylinder (basic calculation)
• mesh (algorithm for finding smallest spherical volume enclosing vertices)

The rest will probably remain using the default e.g. parabola and csg.