Computes x raised to the power y.
An error occurs if x is zero and y
is less than or equal to 0, or if x is negative and
y has a fractional part. Note that
x*x is the same, but is computed faster than
x - any number.
y - exponent
x = pow(100,1); // x is now 100
x = pow(2,8); // x is now 256
x = pow(1.414,2); // x is now 1.999