followup: package and code review for package to prioritize contact follow-up

[mjrolland]

The followup package (ffinger/followup) needs independent code review to make sure
- the code and documentation are accessible to others
- the code does what it is supposed to do
- the way the package is structured and solves the task at hand is sensible

[impact: Contact tracing is an essential part of the COVID-19 response in many countries and of infectious diseases in general. Regular (phone) followup of known contacts to ask them if they have developed symptoms improves can improve the performance of the procedure since contacts who develop symptoms can immediately be directed towards triage and testing facilities, isolation and medical care, decreasing the risk for them to infect others. With high numbers of reported cases the number of contacts in many countries becomes difficult to manage. This package provides a way to objectively prioritize contacts to be followed-up according to the probability that they have yet developed symptoms.]
[originally proposed by @ffinger]
[suggested repo: https://github.com/ffinger/followup | @ffinger]
[additional notes: ]