Ensure all CIDs are positive integers when using pc_prop()

[stitam]

Related to Issue #407.

This PR updates pc_prop() such that:

• All queries are converted to positive integers
• If this produces NA values, the function prints informative messages, if verbose messages are enabled
• Column CID of the final output table is the original vector of queries.

The last point is to ensure downstream processing e.g. merging tables is not impacted.

PR task list:

• [x] Update NEWS
• [x] Add tests (if appropriate)
• [x] Update documentation with devtools::document()
• [x] Check package passed