Address pending Dataset.dims change in data_collections.py

[EdwardSafford-NOAA]

Description

The updated python version on hera now produces this notification of an pending change to Dataset.dims in data_collections.py which will need to be addressed at some point:

0: ds = xr.Dataset({var_name: (coord, dt_arr), 0: /scratch1/NCEPDEV/da/python/envs/eva/lib/python3.10/site-packages/eva/data/data_collections.py:386: FutureWarning: The return type of Dataset.dimswill be changed to return a set of dimension names in future, in order to be more consistent withDataArray.dims. To access a mapping from dimension names to lengths, please use Dataset.sizes.