Add setter for ionization fraction

[wtbarnes]

Similar to #325 and how the abundance is handled, there should be logic for passing in an array of ionization fractions in lieu of using the predefined dataset. By default, this would always just use those tabulated values provided by CHIANTI. There would also need to be a check that this array has the same shape as the temperature array, but this could be done in the setter.

This should be done in tandem with #324