code: function forEachColumn(children, callback) {
    React.Children.forEach(children, function(child)  {
      if (child.type === FixedDataTableColumnGroup.type) {
        forEachColumn(child.props.children, callback);
      } else if (child.type === FixedDataTableColumn.type) {
        callback(child);
      }
    });
  }
docs: Helper method to execute a callback against all columns given the children
  of a table.
  @param {?object|array} children
  Children of a table.
  @param {function} callback
  Function to excecute for each column. It is passed the column.