Period Resampling Master Issue
Various issue surrounding resampling with a `PeriodIndex`
- [x] #12884 Return PeriodIndex if possible
- [x] #12770 recursion error on existing freq (#12874)
- [x] #12774 count (#12874)
- [x] #12868 empty PeriodIndex (#12874)
- [x] #13212 empty dataframe (#13079)
- [x] #11983 with Panels?
- [ ] #7744 convention issues
- [x] #12883 recursion with 'W' freq when upsampling (#12874)
- [x] #13083 ohlc resampling
- [x] #13224 with NaT
- [ ] #14008 MultiIndex with TimeGrouper
- [x] #14213 Verify ``loffset`` works.
- [ ] #15146 Timestamp freq -> Period