section (section) {
        if (!arguments.length) {
            return this._section;
        }
        this._section = section;
        return this;
    }
Get or set the section function for the data grid. The section function takes a data row and
returns the key to specify to {@link https://github.com/d3/d3-collection/blob/master/README.md#nest d3.nest}
to split rows into sections.

Do not pass in a crossfilter section as this will not work.
@example
// section rows by the value of their field
chart
    .section(function(d) { return d.field; })
@param {Function} section Function taking a row of data and returning the nest key.
@returns {Function|DataGrid}