Provide links to references for each model

[steven-murray]

It would be awesome if each chosen component model (Tinker 2008, SMT, etc.) would be able to link to a particular paper that introduced that model. Not sure where exactly to put the link --- probably just as an icon next to the model name, once it is chosen? I intend to include these references in the actual hmf/halomod code. So again, this could be a job for a deploy-script.