add more set-cover references?

[ctb]

turns out another source of set-cover references in bioinformatics (not min-set-cov, but multi-set-cover) are the various diginorm publications -

• https://github.com/openjournals/joss-papers/blob/joss.03790/joss.03790/10.21105.joss.03790.pdf
• https://www.nature.com/articles/s41598-019-41502-9
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5496428/

but I'm not sure how relevant they are to this paper; they might just be distracting. I don't think we are explicitly claiming that our min-set-cov approach is super novel, just that we're applying it here to do cool things. @luizirber @dkoslicki thoughts welcome!