Implement closed-loop mechanisms

[tkoolen]

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