REF: de-lazify Resampler init
AFAICT the only reason `Resampler.__init__` has a lazy component is bc we call `super().__init__(None)` in `GroupByMixin.__init__`.  By avoiding that (and directly setting the one attribute we need that super call to set), we can de-lazify `Resampler.__init__`, which in turn allows simplifying a bunch of other methods