Performance improvement: `AreaDefinition.__getitem__` force to lons/lats array recomputations after slicing

Problem description

Currently, the method AreaDefinition.__getitem__ (L2436 of geometry.py), when performing AreaDefinition slicing, it discards the possible precomputed lons/lats arrays attached to the class instance.
In the case of a large AreaDef (i.e. geostationary), recomputation of lons/lats after a slicing can take several seconds if the sliced domain is quite large.

Proposition of code to add to AreaDefinition.__getitem__

if self.lats is not None: 
     lats = lats[yslice,  xslice]
else: 
     lats = None 
if self.lons is not None: 
     lons = lons[yslice,  xslice]
else: 
     lons = None 

# Pass lats, lons to the AreaDefinition call 
 new_area = AreaDefinition(self.area_id, self.description,
                                  self.proj_id, self.crs,
                                  total_cols,
                                  total_rows,
                                  new_area_extent,
                                  lons = lons, lats = lats) # NEW 

Side note

The same approach could be employed to also avoid the recomputation of the cartesian_coords attribute.

Start in pyresample/geometry.py at AreaDefinition.getitem around line 2436 and inspect how slicing constructs the new AreaDefinition. Check the existing coordinate attributes and validate that slicing preserves already computed values without forcing recomputation; also consider whether the mentioned cartesian_coords case is in scope. Done means sliced areas retain available precomputed coordinates and existing tests still pass.