Provide links to primary publications

[andim]

It would be nice to better link custom pyrepseq functionality to the publications where they were first described.

I can think of at least four papers

• Yuta's Tidytcells paper (used in standardize_dataframe)
• Coincidence analyis paper (introduces the logic of pc and pc Delta)
• Unbiased pc errorbars paper (var_pc and std_pc)
• Touch's SymDel lookup paper (function in pyrepseq.nn)

We might want to add both an overall list with citation information to the Readme and also add links within the relevant docstrings.