additional setting to group repeated citations

[samthiriot]

adds an option in the settings of the plugin to deal with repeated citations (when using tpcite and tpref) #226.

The default behaviour is the current one (one cite = one ref). Grouping means bibtex-like citation with one unique id = one ref.

points of attention:

• requires a change in settings, so in the database. Tested with both installation from scratch and update of an existing installing, but double checking the code is always better.
• double check the naming of the option in the settings page