Add `get_key_columns()` function

[joshwlambert]

This PR addresses #63 by adding a new exported function get_key_columns() which takes in parameter data from {epireview} and returns the key columns with respect to the chosen epidemiological parameter (e.g. CFR or delay distribution).

This was worked on by @ruthmccabe and myself during the {epireview} dev day.

New unit tests have been added for the new function.

• [ ] I have added new dependencies in DESCRIPTION
• [ ] I have bumped the version number in DESCRIPTION
• [ ] I have updated the NEWS.md
• [x] I have added unit tests using testthat
• [ ] I have checked that the package can install locally using devtools::install_local("<path-to-epireview>")
• [x] I have added documentation where required
• [x] My documentation includes examples of how to use the function
• [ ] If necessary, I have updated priority-pathogens

[ruthmccabe]

with this, it would be amazing if we could rebase from the main branch which has the doi/article_info column and add this to the key_cols function (lines 55 - 56)

[joshwlambert]

Good idea. I think it should be rebased onto the develop branch, as this seems to be the most up-to-date.

I've finished implementing everything for this PR, so happy for anyone from the epireview team to take it from here, and rebase or modify this PR in whichever way is best.

[ruthmccabe]

@joshwlambert because this is included in the get_specific_params branch and I am making an update in relation to the comment made by @sangeetabhatia03 on that PR I now propose closing this and including it altogether, but let me know if you disagree!

[joshwlambert]

@ruthmccabe that's fine with me. There are 4 commits on the get_key_cols branch that are not on the get_specific_params branch. These will need to be either merged or manually added if this PR is closed and the changes are made on get_specific_params branch/PR.