loadEdges(tObj, toWL, typeWL) {

    throw new MissingOverrideError(this, 'loadEdges');

  }
Returns all edges stored in the specified tiddler.

@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. If typeWL is not passed it means
    all types are included.
@return {Object<Id, Edge>|null}