Address a few warnings

[zmoon]

These warnings:

tests/test_remap.py::test_combine_da_da
  /home/runner/work/monet/monet/monet/monet_accessor.py:197: UserWarning: rename 'longitude' to 'x' does not create an index anymore. Try using swap_dims instead or use set_index after rename to create an indexed coordinate.
    dset = dset.rename({lon_name: "x", lat_name: "y"})

tests/test_remap.py::test_combine_da_da
  /home/runner/work/monet/monet/monet/monet_accessor.py:197: UserWarning: rename 'latitude' to 'y' does not create an index anymore. Try using swap_dims instead or use set_index after rename to create an indexed coordinate.
    dset = dset.rename({lon_name: "x", lat_name: "y"})

tests/test_remap.py::test_combine_da_da
  /home/runner/work/monet/monet/tests/test_remap.py:98: 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`.
    assert new.dims == {"z": 5, "y": n, "x": n}

tests/test_remap.py::test_combine_da_da
  /home/runner/work/monet/monet/tests/test_remap.py:108: 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`.
    assert a.shape == (model.dims["z"], n), "model levels but obs grid points"