Create bounds for RotatedPole, Creep fill

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR kinda fixes Compute bounds for curvilinear grids #96
• (If applicable) Documentation has been added / updated (for bug fixes / features)
• This PR does not seem to break the templates.
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Add regrid.create_bounds_rotated_pole. Uses cartopy to get lon_bounds and lat_bounds from the Rotated Pole grid mapping. Unique to that projection, but this covers most cases at Ouranos at least... Still waiting for a proper cf-xarray - cartopy link to implement this upstream. Or to have a brilliant idea and fix the estimation in cf-xarray. In the mean time, this is good enough I think.

  • I added checks in regrid._regrid and spatial_mean so that the conservative-rotated_pole-no_bounds case is covered automatically.

• Add xs.spatial for spatial tools. Currently only the creep_fill method to fill missing values with the mean of the neighbours. Can also be used for other purposes like making wider land-sea masks.

• Allow passing GeoDataFrame in spatial_mean's region. This is useful if you want to open a shapefile and subset it before the mean.

Does this PR introduce a breaking change?

No.

Other information:

[aulemahal]

FYI, @mccrayc @marielabonte

[juliettelavoie]

Just add spatial in api.rst to appear in the docs.