Allow for robot variants/joint groups

**Is your feature request related to a problem? Please describe.**
As experienced in #57, adding the same robot with a different planning space is currently a bit silly and requires significant duplication. This is partially because the scripts make assumptions about where robot URDFs and meshes live based on their name, but partially because of the need to compile new `fkcc` for each variant.

**Describe the solution you'd like**
We should support defining robot variants/joint groups that reuse the same meshes, URDFs, and ideally do not require adding an entirely new robot module. The first part of this is easy - we can add resource path information to each robot, rather than relying on a name-based heuristic to look them up. The last part probably requires adding a JIT compiler.