function cidentity( z ) {
	return z;
}
Evaluates the identity function for a double-precision complex floating-point number.

@param {Complex128} z - input value
@returns {Complex128} input value

@example
var Complex128 = require( '@stdlib/complex/float64/ctor' );

var v = cidentity( new Complex128( -1.0, 2.0 ) );
// returns <Complex128>[ -1.0, 2.0 ]