Rename `ioneq` to `ionization_fraction`

[wtbarnes]

"ioneq" is typically used in CHIANTI to denote the ionization fraction in equilibrium and is used as the name of the property on the Ion object to represent this quantity. However, I find the name quite cryptic and not overly descriptive. This should be changed to ionization_fraction. This is straightforward to do, but should be done carefully.