Added docstrings to is_1d and is_2d

[knutfrode]

No description provided.

[gauteh]

I think you need to have them on the same line for them to show up:

        """Returns True if dataset is 1D, i.e. time is a 1D coordinate variable."""

[knutfrode]

Hm, but for e.g. this method, the docstring shows although it is on separate lines, in the exact same way:
https://github.com/OpenDrift/trajan/blob/main/trajan/traj.py#L289
https://opendrift.github.io/trajan/generated/xarray.Dataset.traj.assign_cf_attrs.html

So it is perhaps since is_1/2d are @abstractmethods ?
But these are defined in both traj1d.py and traj2d.py, although just the common abstractmethods seems to be (not) documented.

[knutfrode]

I put the docstrings on a single line, and I also added "show-inheritance": True, in autodoc_default_options according to
https://stackoverflow.com/questions/17841323/how-to-annotate-a-member-as-abstract-in-sphinx-documentation

But none of this seem to make any difference.