Limit use of CustomJoints in distributed models

[aseth1]

CustomJoint is very flexible and intended to model complex joint types, but of course it can be used to model simpler joints like revolute, planar and gimbal joints. However, it is about 2x slower and adds unnecessary complexity to the model (and model file). A PinJoint, PlanarJoint, etc... are more compactly represented and easier to understand.

Newly distributed models, like Rajagopal2015 (#35), in 4.0 should be updated to limit the use of CustomJoints to joints that require coupling between rotational and translational spatial motion (like the knee).

[apoorvar]

On a related note - I remember when building my model that using a Ball Joint vs. a Gimbal Joint for the hip lead to slightly different behaviors in the plotter. Because of the different implementations, there were differences in how moment arm was computed, and I believe there were issues in calculating muscle-tendon lengths. It may be worth adding some documentation somewhere (maybe in the User's Guide) teaching users the difference between the two and when to use which implementation.