BUG: TimeGrouper outputs different result by column order
Merge pull request #6908 from sinhrks/grouper

BUG: TimeGrouper outputs different result by column order
ENH: Implement Panel.pct_change

Had to change the implementation in generic.pct_change
to not pass `periods` as a keyword arg to generic.
Once the signitures of DataFrame.shift() and Panel.shift()
are aligned, this change can be reverted.