function heavisidef( x, continuity ) {
	return addon( x, str2enum( continuity ) );
}
Evaluates the Heaviside function for a single-precision floating-point number.

@private
@param {number} x - input value
@param {string} [continuity] - continuity option
@returns {number} function value

@example
var v = heavisidef( 3.14 );
// returns 1.0

@example
var v = heavisidef( -3.14 );
// returns 0.0

@example
var v = heavisidef( 0.0, 'half-maximum' );
// returns 0.5

@example
var v = heavisidef( 0.0, 'left-continuous' );
// returns 0.0

@example
var v = heavisidef( 0.0, 'right-continuous' );
// returns 1.0

@example
var v = heavisidef( 0.0, 'discontinuous' );
// returns NaN

@example
var v = heavisidef( NaN, 'right-continuous' );
// returns NaN