load_tile_map: Register the rio accessor by importing rioxarray so the returned raster has CRS

[seisman]

Description of proposed changes

The rio accessor is only registered when rioxarray is imported. Currently, we import rioxarray in Figure.tilemap (

pygmt/pygmt/src/tilemap.py

Line 12 in 3a32169

importrioxarray# noqa: F401

) but don't do it in load_tile_map. Thus, the raster returned by load_tile_map alone may have CRS set or not, depending on if a script imports rioxarray or not elsewhere.

This PR fixes the issue by importing the rioxarray package in pygmt/datasets/tile_map.py so that the rio accessor is always registered when rioxarray is installed.

[weiji14]

Have manually triggered the doctests CI for this branch (which has rioxarray installed) at https://github.com/GenericMappingTools/pygmt/actions/runs/9867651036/job/27248357251, looks ok, but a couple of suggestions.

[weiji14]

To be honest, we probably don't need the if hasattr(raster, "rio") check, because the spatial_ref int64 ... 0 line above (added in #3321) already means that rioxarray is used (the doctests CI run with rioxarray installed). But I guess it'll be good to mention rioxarray somewhere in the docstring, albeit indirectly.