Fixed warning about ragged arrays

was using np.size to get length of the z list, but now that we
allow None in the list that caused a warning that the array was
ragged. Switched to using len()

Author: rlabbe

filterpy/kalman/UKF.py

@@ -603,7 +603,8 @@ def batch_filter(self, zs, Rs=None, dts=None, UT=None, saver=None):
                 raise TypeError(
                     'each element in zs must be a 1D array of length {}'.format(self._dim_z))
 
-        z_n = np.size(zs, 0)
+        z_n = len(zs)
+
         if Rs is None:
             Rs = [self.R] * z_n