do not use URI reference for JSON Pointers
change JSON serialization of Identifiable

Always include the field "id" using the identifier's string value. When
the identifier is based on a URI include the optional field "uri" using
the URI's string value.
fix handling of cyclic dependent precursor actions

Filter cyclic dependent precursor actions and create a precursor-less
candidate action provision rather than ignoring a required action
altogether when extending a graph.