function tweenArc(a) {
            let i = d3Interpolate.interpolate(this._current, a);

            this._current = i(0);

            return function(t) {
                return shape(i(t));
            };
        }
Stores current angles and interpolates with new angles
Check out {@link http://bl.ocks.org/mbostock/1346410| this example}

@param  {Object}     a   New data for slice
@return {Function}       Tweening function for the donut shape
@private