add clean_NA_variants function

[sfirke]

Takes a data.frame, turns all instances of "N/A" and "#N/A" and "NA" into true NA values. Maybe it takes a parameter to either clean those exact terms, or grep them, filtering out say "N/A- I have not yet used or received this support/tool."

[sfirke]

It should have list of common NA values like the ones above it uses by default, as well as a parameter to add your own string vector at the function call.

[sfirke]

should it have a "loud" option to print a summary of what it cleaned?

[sfirke]

Consider the option to not use the default set of built in NA strings, for more cautious or flexible uses