Improve Caputo diffusive methods

[alexfikl]

Currently, the diffusive methods for the Caputo derivative require to solve an ODE like

$$\phi'(\xi; \omega) = h(\omega) f^{(m)}(\xi) - g(\omega) \phi(\xi; \omega)$$

where we have the derivative of the function. This currently requires an analytic expression for the derivative, which is not great.. We should use some nice approximation.

This may require some custom ODE solver, so we can have control over the time steps at which to evaluate $f(\xi)$.