cubicweb #3404831 [c-c schema] add option to restrict digram to one etype and its neighbours [validation pending]

prioritynormal
typeenhancement
done in3.19.0
load0.200
load left0.000
closed by#b1ff6525ece3 [c-c schema] option to show single etype (closes #3404831)
patch[c-c schema] option to show single etype (closes #3404831) [applied]