Returns the signum function of the argument; zero if the argument
is zero, 1.0 if the argument is greater than zero, -1.0 if the
argument is less than zero.
Special Cases:
If the argument is NaN, then the result is NaN.
If the argument is positive zero or negative zero, then the
result is the same as the argument.
Parameters:
d
the floating-point value whose signum is to be returned