getReferences(tObj, toWL, typeWL) {

    throw new MissingOverrideError(this, 'getReferences');

  }
Returns a list of tiddlers (= tiddler names) that are targeted by the specified tiddler.
Note: All referenced tiddlers have to be grouped by their edge type.

@interface
@param {Tiddler} tObj - the tiddler that holds the references.
@param {Object<TiddlerReference, boolean>} toWL - a whitelist of tiddlers that are allowed to
    be included in the result.
@param {Object<id, EdgeType>} typeWL - a whitelist that defines that only Tiddlers that are linked
    via a type specified in the list may be included in the result.
@return {Object<string, TiddlerReference[]>|null} a list of referenced tiddlers grouped by their edge type.