Korg vs MARCS electron number density tracking issue

**edit: newer plots are below this post**
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#179 modifies Korg to recalculated the electron number density, rather than taking the value from the model atmosphere at face value.

In general, things look pretty good at the photosphere. "reldiff" here is `(recalculated ne/marcs ne) - 1`. The dissagreement at low logg/high Teff is no big deal since that part of the Kiel diagram is unphysical. Those marcs atmospheres didn't converge and were filled in with RBF extrapolation, so the thing I'm comparing to probably isn't physically self-consistent. For the M dwarfs, I think the problem is the lack of charged molecules, but I'm not certain.

If you look at the worst case atmospheric layer, the match is much worse. Again the worst stuff is happening in the part of the Kiel diagram without many stars (low logg/high Teff). In this regime, the worst disagreement is at the bottom of the atmosphere. It's notable that `calculated ne/marcs ne` is very close to 1, but I haven't been able to come up with a theory of what's going on.

Here's the electron number densities in the lowest layer of the atmosphere calculated by Korg (first) and MARCS (second). The Korg electron number densities have "structure" not found in the marcs values which corresponds to a sharp increase in the temperature at tau=100 (plotted third). This seems likely to be driven by the model atmosphere interpolation.

One last plot: This is `(recalculated ne/marcs ne) - 1` as a function of $\tau$ across the Kiel diagram. The gray lines show the "zero" for each tiny plot, and the red dot marks the photosphere.