Misc. fixes for newer versions of Xarray

These fixes fall into three categories:

1. Xarray's groupby no longer supports squeezing out size 1 dimensions.
   Instead, I've set `squeeze=False` and added explicit calls to `.squeeze()`.
2. Slight differences in dtypes/rounding (I believe these are indirectly due to
   changes required for NumPy 2.0 compatibility).
3. Various cases where type checking is stricter (I've added `type: ignore` to
   these lines)

PiperOrigin-RevId: 671238424

Author: shoyer

weatherbench2/derived_variables.py

@@ -664,7 +664,9 @@ def interpolate_spectral_frequencies(
 
   def interp_at_one_lat(da: xr.DataArray) -> xr.DataArray:
     da = (
-        da.swap_dims({wavenumber_dim: 'frequency'})  # pytype: disable=wrong-arg-types
+        da
+        .squeeze('latitude')
+        .swap_dims({wavenumber_dim: 'frequency'})  # pytype: disable=wrong-arg-types
         .drop_vars(wavenumber_dim)
         .interp(frequency=frequencies, method=method, **interp_kwargs)
     )
@@ -673,7 +675,7 @@ def interp_at_one_lat(da: xr.DataArray) -> xr.DataArray:
     da['wavelength'] = da['wavelength'].assign_attrs(units='m')
     return da
 
-  return spectrum.groupby('latitude').apply(interp_at_one_lat)
+  return spectrum.groupby('latitude', squeeze=False).apply(interp_at_one_lat)
 
 
 @dataclasses.dataclass