Allow "designer friction"

[jrleeman]

Allow a,b, and Dc to vary as a function of slider velocity in some arbitrary functional form provided by the user.

[jrleeman]

This is becoming more important. We need to implement a way for the user to pass a function that defines the rate and state parameters as a function of velocity. This will probably directly tie into #35 as I believe we need the slider velocity to be calculated every step of the integrator or this could get really unstable.