function roundsd( x, n, b ) {
	return addon( x, n, b );
}
Rounds a double-precision floating-point number to the nearest value with `n` significant figures.

@private
@param {number} x - input value
@param {PositiveInteger} n - number of significant figures
@param {PositiveInteger} b - base
@returns {number} rounded value

@example
var v = roundsd( 3.141592653589793, 3, 10 );
// returns 3.14

@example
var v = roundsd( 3.141592653589793, 1, 10 );
// returns 3.0

@example
var v = roundsd( 12368.0, 2, 10 );
// returns 12000.0

@example
var v = roundsd( 0.0313, 2, 2 );
// returns 0.03125