Improve the realism and generality of models of musculoskeletal geometry

[mlprt]

So far, the only muscle models implemented in Feedbax are variants of the Virtual Muscle Model, and their usage is limited to pre-defined muscoloskeletal geometries. For example, MuscledArm has constant moment arms that determine how muscle forces produce torques on the joints of a two-link arm. The values of the moment arms can be changed at construction time, however the number of muscles cannot. In a realistic model, moment arms should also vary with muscle length -- currently, Feedbax does not model this.

MotorNet has a more flexible and realistic approach to modeling musculoskeletal geometry, by allowing the user to define arbitrary wrappings, i.e. attachment points of muscles on the skeleton, and by modeling variable moment arms.

I'd like to follow MotorNet's lead, here.