`PrescribedController` bug fixes

[nickbianco]

Fixes issue #3749, #3750

Brief summary of changes

These changes fix bugs in PrescribedController recently introduced after converting Controller to support a list Socket to maintain actuator connections. This PR also changes the behavior of prescribeControlForActuator() by making a copy of the Function at the passed in Function pointer rather than taking ownership. This change protects users from possible crashes when Function ownership is passed to the controller (e.g., during garbage collection in Java/Matlab).

The main changes:

• A check was added during deserialization to ensure that the number of actuators in the list Socket matches the number of control function. EDIT: removed, this was causing serialization tests to fail.
• The map _actuLabelsToControlFunctionIndexMap is also updated during deserialization to account for any control functions existing the model.
• The functions in ControlFunctions are now reordered based on the order of actuators connected via the list Socket to provide the correct order for computeControls() and serialization. This actually fixes an undetected bug: the member variable _actuIndexToControlFunctionIndexMap was not being used within computeControls() as it should have been. However, reordering ControlFunctions makes _actuIndexToControlFunctionIndexMap unnecessary, so it has been removed.

Testing I've completed

• Added relavant tests to testControllers.cpp.
• Tested the changes on the sample scripts included in the related issues: no crashes occur when passing functions to prescribeControlForActuator.

Looking for feedback on...

• If we're going to copy the Function passed via prescribeControlForActuator(), should change the signature to use a const reference to the function (i.e., const Function& prescribedFunction), rather than a pointer? It would change usage in C++, but scripting users wouldn't notice a difference.

CHANGELOG.md (choose one)

• no need to update because...bug fixes (unless we change the signature for prescribeControlForActuator()).

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This change is 

[nickbianco]

@carmichaelong this is now ready for review.

[nickbianco]

@carmichaelong ready for re-review.

[nickbianco]

Thank you @carmichaelong!