[Moco] Adding support for `PrescribedController` and future controller types

[nickbianco]

Fixes issue #3605

Brief summary of changes

This PR adds support for models with PrescribedControllers in Moco optimizations, and lays some ground work for future controller types that will map OCP controls to Model controls. Therefore, going forward, the relationship between OCP controls and an OpenSim model controls will no longer necessarily be a one-to-one mapping. These changes include support for the two simplest control mapping cases: 1) one-to-one mapping (i.e., current behavior) and 2) controls associated with a PrescribedController are excluded from the OCP.

Changes include:

• Added ControlAllocator which manages a Vector discrete variable containing the OCP controls and provides Output channels to map these controls to OpenSim Model controls.
• Removed DiscreteController in favor of ControlAllocator.
• Added the abstract class InputController. This owns a list Input which concrete implementations can use to take a subset of the Output values from ControlAllocator to be mapped to Model controls.
• Added ActuatorInputController, the simplest concrete implementation of InputController. This controller provides a one-to-one mapping between OCP controls and Model controls. This enables the original Moco behavior for controls, but also supports handling a subset of Model controls (e.g., when a `PrescribedController is present).
• Added options to ignore controlled Actuators in relevant MocoGoals via setIgnoreControlledActuators().
• Various minor drive-by fixes.

Testing I've completed

Added a test for PrescribedController to testMocoInterface.cpp.

Looking for feedback on...

• The name "inputs" for the InputController's list Input. Something less generic would probably be preferred, but I'm having trouble coming up with a term to describe the scalar values that drive an InputController.
• Anything else.

CHANGELOG.md (choose one)

• updated.

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This change is 

[aymanhab]

A drive-by (as called by a colleague), the use of the name allocator would be confusing to anybody who uses containers/c++ constructs that utilize std::allocator (https://en.cppreference.com/w/cpp/memory/allocator) to mean something specific to memory management. Are there other suggestions for the name/alternatives?

[nickbianco]

@aymanhab ControlDistributor would be a reasonable alternative. I waffled between that and ControlAllocator and would be fine swtiching.

[aymanhab]

Love the ControlDistributor name but I'm biased :man_shrugging:

[nickbianco]

I've updated the InputController interface to better support generic control names (i.e., non-actuator control names). I've also made an update where Model control names not included in the trajectories returned by CasADi and tropter are added to the MocoTrajectory returned to the user for convenience. Sorry to expand a bit, but I think this is a pretty essential addition.

Working now on Doxygen documentation.

[nickbianco]

@aymanhab, this is almost ready to merge, did you have anything else you wanted to check/comment on?