Add implicit support to Millard2012EquilibriumMuscle #2711
Description
Is there interest in an implicit version of the 2012 Equilibrium model? If yes, this is something I could start to work on. It should require some modest changes to the curves (so that the gradients don't go to zero) and one additional function.
https://github.com/opensim-org/opensim-moco contains DeGrooteFregly2016Muscle that provides an implicit form and which we use extensively in Moco.
It would be great to also add implicit support to the 2012 Equilibrium model.