'**SUMMARY**
Return a sorted FeatureSet with the longest features first.
**RETURNS**
A sorted FeatureSet.
**EXAMPLE**
>>> img = Image("Lenna")
>>> l = img.findLines().sortLength()
>>> lengt[-1] # length of the 0th element.'

def sortLength(self):

    return FeatureSet(sorted(self, key=(lambda f: f.length())))
