Thelen2003Muscle equilibrium solve failure

[aseth1]

@chrisdembia reported during a developer meeting that Scott Uhlrich encountered this failure only for specific model and state values affecting a single muscle. The failure can be reproduced but it is not clear why the algorithm fails to find an equilibrium since it does not appear to be a singularity in fiber or tendon force production. The Newton solve fails to find appropriate step size and fails due to exceeding the iteration limit. A test case is being added.

[jenhicks]

@aseth1 @mjhmilla OK to close?

[mjhmilla]

Actually I think this one should remain open. Here's why:

Problem: Several muscle models have the possibility of failing to initialize correctly.

Solution Steps:

1. [Done] Build a general purpose initialization test that can catch all of the various ways a model can fail to initialize. This has been done and the code is currently sitting in the branch feature_equilibrium.
2. [Open] Fix the initialization methods for the models that have initialization problems.

I have a straight forward fix (remove all Newton methods and replace with bisection + initialize the model so that the tendon is static). I simply haven't gotten to this yet. If there is a date when you'd like to see part 2 completed please let me know.

[jenhicks]

Closing as the (important) remaining issues are covered elsewhere.