add hyperedges

[cassinius]

• class hyper-edge spanning 3 defined node classes
• has an internal definition of its 3 connected node types
  => implicitly via the node objects it is handed..
• has an internal definition of its 3 connecting edge types
• HE takes a hypernode feature object and an info edge features object
• decouple entity from ArangoNode, derive from entity & define own CRUD methods on it
• transactions? rollback?
• test cases?

[cassinius]

• Re-structure base classes, so that we can use a custom object type as the create methods argument
  • don't violate Liskov's substitution principle...
  • static async create<C extends HE3CreateCfg, T extends ArangoHyperEdge3>(cfg: C): Promise
    => DONE by introducing a BaseDoc class which derives from Entity and implements CRUD methods for Nodes & Edges, but not for HyperEdge

[cassinius]

right now, we cannot query an edge by _from & _to, combined; this is a problem with duplicate edge creation, which at the moment only leaves us the choice to -) use create & either catch the Error or ignore it... introduce either:

• uniqueAttr: [_from, _to] for all edges, so we can use upsert => but do we want that in any case within hyperedges??
  => no!
• write an edge-specific byNodes query which takes _from & _to as arguments, so we can implement it in a 2-step process