function quantile( arr, p ) {
	var len = arr.length;
	var id;
	var h;
	h = ( ( len - 1.0 ) * p ) + 1.0;
	id = floor( h ) - 1.0;
	return arr[ id ] + ( ( h - floor( h ) ) * ( arr[ id + 1 ] - arr[ id ] ) );
}
Computes a quantile of the values in a numeric array.

@private
@param {NumericArray} arr - sorted 1d array
@param {Probability} p - quantile prob [0,1]
@returns {number} quantile