code: def unit_tangent(self, T):
          
          seg_idx, t = self.T2t(T)
          return self._segments[seg_idx].unit_tangent(t)
docs: returns the unit tangent vector of the Path at T (centered at the
          origin and expressed as a complex number).  If the tangent vector's
          magnitude is zero, this method will find the limit of
          self.derivative(tau)/abs(self.derivative(tau)) as tau approaches T.