Add head(), tail() and thin() methods?

These would be shortcuts for `isel`/slice syntax:
- `ds.head(time=5)` -> `ds.isel(time=slice(5))`: select the first five time values
- `ds.tail(time=5)` -> `ds.isel(time=slice(-5, None))`: select the last five time values
- `ds.thin(time=5)` -> `ds.isel(time=slice(None, None, 5))`: select every 5th time value
