[dataimport] add a filter function to not fail if some extentity has several values for an attribute of final relation the function will simply record a warning in the import log and keep a value randomly. Notice the function has to be explicitly inserted in the transformation stream.

usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py