[Feature Req] Simle way to generate citations

[steven-murray]

Is your feature request related to a problem? Please describe.
Would be awesome if you could do something like mf = MassFunction(); mf.citations and print a list of the references to cite that go into your particular model.

Describe the solution you'd like
Probably the way to do this is to have each model for each component have an attribute like citations, and then use the framework to bring them all together. It could even intelligently determine which models have actually been used in calculations so far.