'**SUMMARY**
Returns a new FeatureSet, with the smallest y coordinates features first.
**RETURNS**
A featureset sorted based on area.
**EXAMPLE**
>>> img = Image("lenna")
>>> feats = img.findBlobs()
>>> feats = feats.sortY()
>>> print feats[-1] # biggest blob
>>> print feats[0] # smallest blob'

def sortY(self):

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