_checkOptions(options = {}) {
    if (options.clusterEdgeProperties === undefined) {
      options.clusterEdgeProperties = {};
    }
    if (options.clusterNodeProperties === undefined) {
      options.clusterNodeProperties = {};
    }

    return options;
  }
This function checks the options that can be supplied to the different cluster functions
for certain fields and inserts defaults if needed
@param {object} options
@returns {*}
@private