Add SynergyController for modeling muscle synergies

[nickbianco]

Add a new class SynergyController (derived from Controller) that enables modeling muscle synergies in OpenSim. This class will leverage OpenSim Inputs to convert synergy activations from any type of input signal sources (an NLP control, an OpenSim Function, etc) into muscle activations.

This class should have properties defining synergy weights and settings to specify how to normalize each synergy vector. Other properties could be used to define scale factors for each synergy, if useful. Methods to compute normalization error will also be included, along with associated Outputs.

This issue is blocked by #3605, since compatibility with Moco must be available before implementation. However, the use of SynergyController will be generic to any OpenSim and not strictly limited to Moco.

[nickbianco]

FYI @bfregly, I've opened this issue to track the development muscle synergy support in OpenSim.