'**SUMMARY**
Computes the variance of the signal.
**RETURNS**
The variance of the LineScan object.
**EXAMPLE**
>>>> ls = img.getLineScan(x=10)
>>>> var = ls.variance()
>>>> var'

def variance(self):

    mean = (float(sum(self)) / len(self))
    summation = 0
    for num in self:
        summation += ((num - mean) ** 2)
    return (summation / len(self))
