'Return a sorted FeatureSet with the features closest to a given angle first.
Note that theta is given in radians, with 0 being horizontal.
**RETURNS**
An array of angle values corresponding to the features.
**EXAMPLE**
>>> img = Image("lenna")
>>> l = img.findLines()
>>> l = l.sortAngle()
>>> print angs'

def sortAngle(self, theta=0):

    return FeatureSet(sorted(self, key=(lambda f: abs((f.angle() - theta)))))
