[autoform] Make RelationField.fromcardinality a class method factory Instead of having the "RelationField" class hard-coded in this factory method (thus preventing any subclass to properly use it), we make that method a classmethod and instantiate the actual class instead of the base one.

#!/bin/sh -e

if [ "$1" = "purge" ] ; then
        rm -rf /etc/cubicweb.d/
        rm -rf /var/log/cubicweb/
        rm -rf /var/lib/cubicweb/
fi

#DEBHELPER#
 
exit 0