ENH: Add circular statistics methods for DataFrames and Series
Let me know if this is out of scope, but it would be nice to perform typical statistics calculations on circular quantities with a similar interface as, e.g., `mean` --> `circmean`, including the `level` arg. It would also be great to be able to note that the input is in degrees to avoid having to convert back and forth from radians.

```python
mean_dir = df.direction_deg.circmean(degrees=True, level="time")
```
