Add a path constraint that requires tendon force to be positive

[chrisdembia]

Fiber damping helps improve convergence but can cause negative forces. We can add a path constraint that avoids negative forces (by causing activation to increase).

Work on this has been started in: https://github.com/opensim-org/opensim-moco/tree/nonnegative_muscle_forces

Solving this requires that we fix the time points at which we enforce path constraints.

#579