'**SUMMARY**
Return a numpy array of the length (longest dimension) of each feature.
**RETURNS**
A numpy array of the length, in pixels, of eatch feature object.
**EXAMPLE**
>>> img = Image("Lenna")
>>> l = img.findLines()
>>> lengt = l.length()
>>> lengt[0] # length of the 0th element.'

def length(self):

    return np.array([f.length() for f in self])
