are we filling y_e in the burn_state that get's passed to the nets?

[zingale]

If we are using tabular weak rates, then we need burn_state.y_e.

For the Strang code path, it looks like we are filling this in update_thermodynamics.

But what about the SDC code path? It is not clear where it is filled.

We should add an assert on it in the pynucastro nets just as a check.