'**SUMMARY**
Returns the longest side of the quandrangle formed by the boundary points.
**RETURNS**
A floating point length value.
**EXAMPLE**
>>> img = Image("mycode.jpg")
>>> bc = img.findBarcode()
>>> print bc[-1].length()'

def length(self):

    sqform = spsd.squareform(spsd.pdist(self.points, 'euclidean'))
    return max(sqform[0][1], sqform[1][2], sqform[2][3], sqform[3][0])
