PERF: declare contiguity in libgroubpy
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] 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

this provides the speedup yes? > this provides the speedup yes?

yes, ported from the \<update\>discussion in\</update\> 1d-version PR and applied to everywhere applicable in this file thanks!