getReferencesFromField(tObj, fieldName, toWL) {

    throw new MissingOverrideError(this, 'getReferencesFromField');

  }
Returns all references to other tiddlers stored in the specified tiddler.

@interface
@param {Tiddler} tObj - the tiddler that holds the references.
@param {String} fieldName - the name of the field to get the reference from.
@param {Object<TiddlerReference, boolean>} toWL - a whitelist of tiddlers that are allowed to
    be included in the result.
@return {Object<Id, Edge>|null}