SDC integrators don't support nspec_evolve != nspec

[zingale]

The SDC integrators don't currently handle how we update species where nspec_evolve < nspec. Siince these still have advective terms, we still would need to do some integration. But the current update_unevolved_species mechanism probably is not enough

[dwillcox]

Ah, this would explain the issue @ajnonaka reported here - AMReX-Astro/MAESTROeX#124

When we update_unevolved_species we are still "evolving" those species, just analytically using the other species values.

We could fix this by:

• write a subroutine unevolved_rhs that returns the analytic RHS for the unevolved species by differentiating the analytic update equation in update_unevolved_species and writing it in terms of the RHS for the evolved species.

• add ifdefs for simplified SDC to network_rhs to call unevolved_rhs in addition to actual_rhs

• add analytic RHS for unevolved species to their advective source terms to get their SDC RHS

[zingale]

we've eliminated nspec_evolve