Future warning `Dataset.dims`

[rogerkuou]

In: https://github.com/TUDelftGeodesy/stmtools/pull/71/files#diff-0b3352e5838067dc55b6b083d1385268ce032917d9d0d7cac6f5d7bdf1c09913

%%timeit -o
# Time subset operation on chunked STM.
stmat_chunked_subset = stmat_chunked.stm.subset(method='polygon', polygon=path_polygon)
phase = stmat_chunked_subset['phase'].compute()

/storage/MobyLe/stmtools/stmtools/stm.py:134: 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`.
  if dim not in self._obj.dims.keys():