Making attribute names more precise: obsdim->obs_dimname, timedim->time_varname

[knutfrode]

So far mainly a mechanical search & replace
Thus some comments are still inconsistent, e.g. refering to time_varname as time dimension.

Also remains to add an attribute trajectory_dimname, but can perhaps be made in a subsequent pull request. So far this is hardcoded as "trajectory" at some places.

[gauteh]

Should we use dim and not dimname in the public API? I think that is what xarray is doing: https://docs.xarray.dev/en/latest/generated/xarray.Dataset.reduce.html#xarray.Dataset.reduce

[knutfrode]

Hm, ok yes, we should aim to be consistent with Xarray.
Shall I then replace all obs_dimname with obs_dim ?

But time_varname I guess is ok, as time_var could be interpreted both as the name of the time variable or as the variable itself. For dimension we do not have this dual possibility.

[gauteh]

Yes, I think this is the best compromise?

[knutfrode]

This an example of why old names were not precise enough.
Thus Time dimension has shape greater than 2 should now be changed to the more precise Time variable has more than 2 dimensions

[gauteh]

Yes, also this probably needs to be: ds[self.time_varname] without the internal quotes.