Rename or alias accessor to tr or ta?

When doing stuff like:

```
ds.traj.iseltime([0, -1]).traj.scatter()
```

to plot first and last observation, writing .traj gets tedious. `cf-xarray` uses only `.cf`, maybe we should rather use `.tr` or `.ta` or `.tj` ? that fits with when we do:

```
import trajan as ta
ds = ta.from_dataframe(....)
```