Add function(s) for continuum lowering

[lemmatum]

There some equations for approximating atomic energy level shifts due to continuum lowering - we should add these. I believe David Salzmann's Atomic Physics in Hot Plasmas has some simple equations which we could start with. Such functions would be handy especially for dense plasmas.

[lemmatum]

Section 3.2 in Salzmann (pg 58) is where continuum lowering and line shifts starts.

[lemmatum]

Paper by B. J. B. Crowley which provides a good reviews on Stewartt-Pyatt, plus accounts for relaxation effects which may have been seen in relatively recent continuum lowering measurements.

CHEMEOS paper has a different continuum lowering model, which is used by some codes at LANL. It includes changes to the statistical weights within a charge state.