pep8, removed duplicate definition of CentralWindow class
Changed ZCA.fit() to optionally compute self.inv_P_. Changed ZCA_Database constructor to only compute ZCA.inv_P_ if necessary. pep8.
changed ZCA so that the choice of whether to store the inverse is made in the constructor, not in fit(). Did this because often fit() isn't called directly; it's called by dataset.apply, which doens't know about this option.
ZCA now attempts to use GPU for large matrix multiplications when possible. Falls back to CPU on MemoryError.
pep 8
Merge pull request #511 from SuperElectric/zca_efficiency

ZCA efficiency improvements