Initial fully functioning package

[pratikunterwegs]

This PR is from a branch off tim (by @thimotei), and makes these changes over that branch, and over main:

1. Standard package infrastructure - NAMESPACE, DESCRIPTION, Rbuildignore etc. (by @thimotei), and some extra package infrastructure - GHA workflows, more linter options, link to Codecov (by @pratikunterwegs);
2. Adds testing infrastructure with high test coverage and a GHA workflow for code coverage,
3. Adds the Ebola 1976 outbreak data to the package, simplifying the examples,
4. Moves the ebola CFR analysis script to the README, and uses a .Rmd README,
5. Adds input checking to most functions,
6. Simplifies some functions (e.g. static_cfr) to return named vectors rather than dataframes,
7. Maintains functionality of format_cfr_neatly to work with named vectors where changed,
8. Removes the epiparameter, reshape2, and lubridate direct dependencies; epiparameter is now suggested, and data.table is added as more useful than reshape2 and lubridate while still reducing one dependency,
9. Adds revised function documentation.
10. Keeps the existing covid-19 USA example, but using new functions rolling_cfr() and new plotting function.

[pratikunterwegs]

Thanks @adamkucharski and @Bisaloo for your input - I've made a few changes to sort the technical issues, but the potential issue with the CFR correctness remains. I'm not able to determine where any error might be as this is primarily an adaptation of Tim's code. Happy to add more tests for algorithmic correctness if these could be suggested, and that might help tackle the issue.

[pratikunterwegs]

Refreshing this review thread so we can merge this added functionality into main