add the explicit Patankar-Euler-Deflhard integrator

[zingale]

This paper: https://arxiv.org/abs/2110.08035 descirbes a stable explicit integrator, Patankar-Euler-Deflhard. It appears first order, but they show how to use Richardson extrapolation with it. This may be an alternative to RKC, or at least it would be good to see how they compare.

[zingale]

it seems that only the species equations fit into the Patankar-Euler decomposition. So we would need to do the energy evolution using a first-order explicit update. It is also not clear how reactions with like nuclei fit into the form, e.g.,

dY(C12)/dt = - ρ Y(C12)**2 N_A <σv>

or triple-alpha?