xarray: Use Dataset.sizes instead of Dataset.dims

[samsrabin]

There are some places in the Python scripts where we use the dims member of Xarray Datasets in order to get dimension lengths. As of the latest ctsm_pylib (Python 3.13.2, Xarray 2025.1.2), this produces a warning (here produced from ctsm5.3.063):

modify_fsurdat.py:49: FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.
  self.numurbl = self.file.dims["numurbl"]

We should go ahead and convert all such instances to Dataset.sizes so we don't have confusing failures when we eventually update Xarray to a version with the new behavior.

[ekluzek]

Adding next to discuss this as a habit to start doing.

[samsrabin]

It would be pretty easy to find and replace all existing instances in our Python code. Before doing that, we could also change our Python testing to turn these warnings into errors (with the warnings filter), so we don't add any more (tested) instances of this behavior.