Refactor PICMI species constand dependency retrieval

[BrianMarre]

In the review of PR #5007, here chillenzer identified the constant dependency retrieval functions as overly long and suggested to replace them with something along the lines of

from functools import reduce
return reduce(set.union, (model.get_constant_dependencies() for model in self.ionization_model_list), set())

Originally posted by @chillenzer in #5007 (comment)