Set inv_P_ attribute for unpickled ZCA

Old unpickled ZCA had no inv_P_ attribute. It is now set
in __setstate__ when object is unpickled.
Remove calls to inv_P_ outside of ZCA

ZCADataset was using ZCA's private attribute inv_P_ directly. The test
and computation of inv_P_ is moved to ZCA.inverse() function, the only place
where inv_P_ is used.