Always use dim argument

[OriolAbril]

I have just seen that xarray has recently (this May) undergone an effort to standardize the dim/dims/dimensions arguments to always use dim. I think we should do the same to keep things consistent and avoid confusion (with ourselves being the main confused people too).

The only places were dims is still used are constructors such as xarray.DataArray(..., dims=). Which is the agreed on convention.

Extra: should this also affect sample_dims->sample_dim in rcParams and batteries included plots?

Ref: https://docs.xarray.dev/en/latest/whats-new.html#id16

[OriolAbril]

We tentatively agree to follow xarray and always use dim instead of dims. However, sample_dims being a different concept will continue to be sample_dims not sample_dim cc @aloctavodia. We'll see how we sleep on the decision