more input type warnings

[atn38]

Minor issue: when I accidentally gave citation_search a data.frame as input, it gave this non informative error

Error in curl::curl_fetch_memory(x$url$url, handle = x$url$handle) : 
  URL using bad/illegal format or missing URL

Should be fairly trivial to type check inputs and give appropriate warnings IMO

[mbjones]

Thanks @atn38 for the feedback, and yeah I agree. We'd welcome a pull request if you had interest in this improvement. If not, we'll try to incorporate this the next time we have cycles to tackle the package.

[jeanetteclark]

I went ahead and added a very simple warning in the main function which I think accomplishes this, though we could make it more robust. Commit abdbd8c