add tag processor

[damian0604]

There is a processor that adds a random field to documents matching a query, and another one for adding a random field. Let's make a processor `tag' that attaches a specific tag (e.g., a project name) to all docs fitting the query.

https://github.com/uvacw/inca/blob/development/doc/howto_annotate.md
https://github.com/uvacw/inca/blob/development/inca/processing/randomfield_processing.py