Read-in abundance consistency

[kslong]

For most of the Sedona/Python, I have been reading in fixed ionization values where H was the only value given and it was fixed at complete ionization. But the data file for elements/ions inadvertently had both H and He available. While models calculated in this manner only had H included in the radiative transfer. The number density of H atoms in these models was less than for a pure H plasma, which resulted in a lower total electron density than I expected.

There are two possible options to make things clearer:

• The program could be modified to simply provide a warning when there are atoms in the elements/ions file that have no abundances
• One could actually modify the program to exclude elements not contained in the fixed abundances file.

In the absence of comments from anyone (@jhmatthews) I plan to just add comments and document the issue. This preserves flexibility, and most of the time, I think when we use fixed abundances we are diagnosing a problem that has somehow occurred, and we would not want the density of ions in the wind to change as we explore different fixed abundance inputs.

[jhmatthews]

I think a warning is the minimal thing to do here and adding comments seems reasonable, but note also the issue raised by @lazygun37 on this before which already documents some issues: #291

[kslong]

I have added warnings both to read the docs and the code itself.