It'd be nice to also test periodicity directly. Something similar to     ```  In [14]: p1 = PPoly([[2], [1]], [0, 1])    In [15]: p1([0.5, 1.5])  Out[15]: array([ 2.,  4.])    In [16]: p2 = PPoly([[2], [1]], [0, 1], extrapolate='periodic')    In [17]: p2([0.5, 1.5])     # 1.5 is 0.5 + period(=1)  Out[17]: array([ 2.,  2.])  ```