code: function getVersion(obj, key) {
    if (obj.metadata === undefined) {
      return undefined;
    }
    return obj.metadata[key];
  }
docs: This variant of getVersion() differs as it allows
  for undefined values to be returned.
  @param {Object} obj The state.map object
  @param {Object} obj.metadata The state.map.metadata object
  @param {string} key One of 'bnd:layer-version', 'bnd:source-version', or 'bnd:data-version'.
  
  @returns {(number|undefined)} The version number of the given metadata key.