REF: Prep for moving Series.apply into apply
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Will be moving Series.apply into apply next. The plan is to have a base class Apply which is parent to both FrameApply and SeriesApply.