'**SUMMARY**
Computes the standard deviation of the signal.
**RETURNS**
The standard deviation of the LineScan object.
**EXAMPLE**
>>>> ls = img.getLineScan(x=10)
>>>> avg = ls.mean()
>>>> std = ls.std()
>>>> plt.plot(ls)
>>>> plt.axhline(y = avg)
>>>> plt.axhline(y = avg - std, color =\'r\')
>>>> plt.axhline(y = avg + std, color =\'r\')
>>>> plt.show()'

def std(self):

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