Rename or alias accessor to tr or ta?

[gauteh]

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(....)