Remove hard coded frames, retrieve from parameters

[kamiradi]

          we'll need to figure out what exactly to weld here, since `"panda_link0"` is, of course, hard-coded. I'm sure there's a way to just weld the base-most link name of the robot description. That, or we can make it a parameter if needed, but hopefully isn't necessary.

Originally posted by @sea-bass in #8 (comment)