[states] Fixes #256. Remove global mutable variables from statemultibody class
As reported in #256 (closed), there was a race condition occuring with the ddp solve. It turns out that it was because we defined data inside the state variables, and it was being accessed by multiple threads.
THis PR corrects this bug.
Edit:
- WIP status: As discussed, I'll make functions of state and actuation const.
- CI integration: to be dealt with by @cmastall
Edited by Rohan Budhiraja