Fix rows with a covidence id being associated with multiple ids

[sangeetabhatia03]

This PR fixes #56 I have included extensive documentation on why this issue arises. Fixed it here to make things work for existing data sets but going forward I think we should ensure that a covidence_id is not associated with multiple ids in priority-pathogens pipeline.

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

[cm401]

Wonderful - this fixes the issues I saw with the Lassa data. (and works fine for Marburg)