ENH: PeriodIndex resampling redux
As discussed here: https://github.com/pydata/pandas/pull/12874, some ideas:
- ~Only return `PeriodIndex`s when resampling `PeriodIndex`~ (#16153)
- Don't use `convention` when doing simple upsampling & downsampling ('simple' meaning a one to many relationship between subperiods and superperiods)
- Define how to handle 'complicated' resampling (meaning a many to many relationship between subperiods & superperiods, such as Week->Month)

ref: https://github.com/pydata/pandas/issues/7744, https://github.com/pydata/pandas/issues/12871, https://github.com/pydata/pandas/pull/8707, https://github.com/pandas-dev/pandas/issues/15944

EDIT: strikethrough above after completion