Boxes and Spheres as Collision Shapes

This pull request adds boxes and spheres as options for collision shapes. Boxes are similar to rods, and are constructed with a height and width instead of a radius. Spheres are specified by a node. These, along with tgRod, now have a common rigid base class. Usage examples are provided in dev/btietz.

This partially addresses issue #11. It should be possible to conclude that issue with one more tgModel derivative, which builds both rods and spheres at nodes within its setup function. Additional modifications to tgcreator should not be required.

Lastly, this branch addresses a few todos in tgcreator, by moving code into the base class tgRigidInfo.