Unify the equilibrium solver for Thelen2003 and Millard2012Equilibrium Muscles #1736
Description
Both Millard2012EquilibriumMuscle and Thelen2003Muscle implement very similar muscle force equilibrium solvers using Newton's method. The differences were further diminished by PR #1728, which patched both implementations to address #1700.
The challenge is mainly one of defining an interface to resolve the different naming conventions since the two solvers use different local methods to evaluate their respective muscle curves and to compute the partial derivatives of muscle and tendon force.