Use %in% to compare strings to gracefully deal with NAs

[sangeetabhatia03]

filter_df_for_metamean was creating rows with all NAs because we used == to compare the column paramter_value_type with Mean/Median etc. Where the column itself was NA, using == led to NA instead of a logical. I have now fixed this so that we use %in% operator to compare strings.

• [ NA] 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>")
• [ NA] I have added documentation where required
• [ NA] My documentation includes examples of how to use the function
• [NA ] If necessary, I have updated priority-pathogens

[CosmoNaught]

LGTM + passes tests. Merging