To extend the publications fields you need to do the following:

(from example: adding DOI fields as full-status fields to the database)

* add new field as column to database (table publications) 
    = make a new checkschema function (see DOI example)
    = in that function, add the new column
    = in that function, if necessary, initialise for all entries the new column (the doi example takes the initial value from the userfields DOI entry, if any)
* extend the publication displays to display the new field if needed
    = in summary views
    = in full publication display
* add the new field to the allowed-fields-per-entry-type (HOW?),
    --> OR add the new field to the publication form directly?
* make sure that those values from the edit form are actually committed to the database if filled in (HOW? is any special action needed if new field added to the allowed-fields-per-entry-type?)
* add the new field to the export functions, to make sure that it will be exported to bibtex

