Add exponential `Force` component for modeling joint limits

[nickbianco]

This component would apply generalized force based on the expressions described in @fcanderson's thesis, which take the form:

$$F_{lig} = k_0 + k_1 e^{k_2 (q - \theta)} + k_3 e^{k_4 (q - \phi)}$$

where $k_0$, $k_1$, $k_2$, $k_3$, $\theta$, and $\phi$ define the torque-angle relationship for a given coordinate, $q$.