Add ExponentialCoordinateLimitForce

[nickbianco]

Fixes issue #4164

Brief summary of changes

Adds ExponentialCoordinateLimitForce, a force element for enforcing coordinate limits using exponential spring functions. Based on the model used by Anderson and Pandy (1999) for modeling ligament forces:

Anderson F.C. and Pandy M.G. (1999). A dynamics optimization solution for vertical jumping in three dimensions. Computer Methods in Biomechanics and Biomedical Engineering 2(3):201-231.

Testing I've completed

Updated testForces.cpp to add tests for ExponentialCoordinateLimitForce.

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CHANGELOG.md (choose one)

• updated.

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