load_map_tiles: Allow a new parameter to control the projection of the returned DataArray

[seisman]

Originally proposed in #2125 (comment)

Perhaps we should convert the returned image to longitude/latitude because it's more commonly used than the Spherical Mercator coordinate system, following https://contextily.readthedocs.io/en/latest/warping_guide.html.

Most tiles are originally served as Web Mercator (EPSG:3857), and reprojecting to latlon (EPSG:4326) would result in distortion. I'd prefer the reprojection to be a user controlled step (e.g. using rioxarray's .rio.to_crs), because if latlon isn't what the user wants, there would be double reprojection (EPSG:3857 -＞EPSG:4326 -＞EPSG:????) which is less accurate than single reprojection (EPSG:3857 -＞EPSG:????).

I'd prefer the reprojection to be a user controlled step (e.g. using rioxarray's .rio.to_crs)

What about having an option so that users can decide on the desired projection and don't have to learn the syntax of the rioxarray package?

I think we can add a new parameter like crs to control the projection. crs should be default to EPSG:3857. If a different CRS is given, then we can use contextily.warp_tiles to reproject the tiles.

With this new parameter, the following lines in Figure.tilemap can be simplified:

pygmt/pygmt/src/tilemap.py

Lines 141 to 145 in 68a17a0

	# Reproject raster from Spherical Mercator (EPSG:3857) to lonlat (OGC:CRS84) if
	# bounding box region was provided in lonlat
	if lonlat and raster.rio.crs == "EPSG:3857":
	raster = raster.rio.reproject(dst_crs="OGC:CRS84")
	raster.gmt.gtype = 1 # set to geographic type