Manage RotatedPole defaults, fix typing (#455)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #xyz
- [x] (If applicable) Documentation has been added / updated (for bug
fixes / features).
- [x] (If applicable) Tests have been added.
- [x] This PR does not seem to break the templates.
- [x] CHANGELOG.rst has been updated (with summary of main changes).
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added.

### What kind of change does this PR introduce?

* `central_rotated_longitude` is kept as None if it is not in the
dataset attributes.
* Attributes are cast as floats before `ccrs.RotatedPole`.

### Does this PR introduce a breaking change?


### Other information:

CHANGELOG.rst

@@ -36,6 +36,7 @@ Bug fixes
 * ``unstack_dates`` now works correctly for yearly datasets when `winter_starts_year=True`, as well as multi-year datasets. (:pull:`450`).
 * Fix ``xs.catalog.concat_data_catalogs`` for catalogs that have not been search yet. (:pull:`431`).
 * Fix indicator computation using ``freq=2Q*`` by assuming this means a semiannual frequency anchored at the given month (pandas assumes 2 quarter steps, any of them anchored at the given month). (:pull:`431`).
+* ``create_bounds_rotated_pole`` now uses the default value if the dataset has no `north_pole_grid_longitude` attribute, instead of crashing. (:pull:`455`).
 
 Breaking changes
 ^^^^^^^^^^^^^^^^

src/xscen/regrid.py

@@ -370,12 +370,13 @@ def create_bounds_rotated_pole(ds: xr.Dataset):
     rlonv = rlonv1D.expand_dims(rlat_vertices=rlatv1D).transpose(
         "rlon_vertices", "rlat_vertices"
     )
-
+    central = ds.rotated_pole.attrs.get("north_pole_grid_longitude")
+    central = float(central) if central is not None else None
     # Get cartopy's crs for the projection
     RP = ccrs.RotatedPole(
-        pole_longitude=ds.rotated_pole.grid_north_pole_longitude,
-        pole_latitude=ds.rotated_pole.grid_north_pole_latitude,
-        central_rotated_longitude=ds.rotated_pole.north_pole_grid_longitude,
+        pole_longitude=float(ds.rotated_pole.grid_north_pole_longitude),
+        pole_latitude=float(ds.rotated_pole.grid_north_pole_latitude),
+        central_rotated_longitude=central,
     )
     PC = ccrs.PlateCarree()