cubicweb-dataio #3478884 Mutualize code in add_triple [validation pending]

prioritynormal
typebug
done in0.4.0
load0.100
load left0.000
closed by#61a1a86fefdd [interfaces] Add_triple is no more abstract but call do_add_triple which is abstract, closes #3478884
patchadd_triple is no more abstract but call do_add_triple which is abstract [applied]