Add calculation of dielectronic level populations

[jwreep]

I don't see another issue for this, so making a new one. See the discussion here.

Fiasco does not currently have a method to calculate level populations for a dielectronic ion.

[jwreep]

I think it's worth clarifying what is meant in this issue since I didn't describe it in particular detail. There are no "dielectronic ions" in reality, but CHIANTI creates a separate model ion for dielectronic excitation to calculate satellite lines. The database includes directories like "fe_17" and "fe_17d" to distinguish the two.

This is described in Section 2.2 of the fourth CHIANTI paper: