Implement closed-loop mechanisms

[tkoolen]

• [x] Add constraint force subspace function for each joint type.
• [x] Store loop joints in Mechanism
• [x] Modify attach! to handle loop joints.
• [x] Enforce loop joint constraints in dynamics.
• [ ] Initial resolution of loop joint constraints using a nonlinear program.
• [ ] Constraint projection
• [x] ~~Consider using a Graphs.jl Graph as the main data structure used in Mechanism.~~ decided to roll my own.
• [x] Modify reattach! etc. to handle loop joints.
• [x] constraint stabilization