Allow specifying the Stewart-McCumber parameter in JJ models

[dolphingarlic]

When working with overdamped Josephson junctions, it's often convenient to specify them in terms of their Stewart-McCumber parameter $\beta_c$ instead of their shunt resistance(s). Would it be a good idea to allow us to specify $\beta_c$ when declaring a model? $\beta_c$ is related to the other model parameters as:
$$\beta_c = \frac{2\pi C R^2 I_c}{\Phi_0}$$