Should we automate the addition of default cf attributes when these are missing? Or should this be something the user has to call in manually? I.e. the addition of the attributes as done by:

ds = ds.traj.assign_cf_attrs()

Could look for the existence of the requested attributes in the class __init__ when a Traj object is created for example, and call this automatically if the attributes are not there? Any thoughts about this @gauteh @knutfrode ? :)