code: function convertEntitiesToJDL(entities, jdl) {
    if (!entities) {
      throw new Error('Entities have to be passed to be converted.');
    }
    init(entities, jdl);
    addEntities();
    addRelationshipsToJDL();
    return configuration.jdl;
  }
docs: Converts the JSON entities into JDL.
  @param entities the entities in plain JS format.
  @param jdl {JDLObject} the base JDLObject to use as conversion result.
  @returns {*} the updated JDLObject.