Minimal env yml, change urdfpy to urchin

[williamito]

Hi! I was interested in trying your implementation of IK for the SO-101 but couldn't run things as-is on MacOS. I think this fixes things?

Changes:

• Edit environment.yml to be named controls_env again and only include dependencies required by the library.
  • The current file appears to be an export from Linux that includes os-specific and incidental dependencies.
• Update dependency and code to use urchin instead of urdfpy. urdfpy is no longer maintained and requires old versions causing conflicts with other packages. urchin is a community-maintained fork mentioned in the issues of urdfpy and seems to be the best current option?

Testing:

• Ran unit test script and the 2 main scripts and saw expected outputs.

Caveats:

• Haven't actually tried the library yet, so hopefully this is sufficient.
• Pruned the dependencies manually, possible I cut something that just wasn't used in the unit tests.