Let pygmt know about geographic grids

Previously it thought every xarray.DataArray grid was Cartesian! So the load_earth_relief() grid passed to GMT was treated as a Cartesian virtualfile, whereas the actual `@earth_relief` file was properly read as Geographic.

Co-authored-by: Dongdong Tian <[email protected]>

Author: weiji14

pygmt/clib/session.py

@@ -1242,7 +1242,7 @@ def virtualfile_from_grid(self, grid, registration="GMT_GRID_PIXEL_REG"):
         gmt_grid = self.create_data(
             family,
             geometry,
-            mode="GMT_CONTAINER_ONLY",
+            mode="GMT_CONTAINER_ONLY|GMT_GRID_IS_GEO",
             ranges=region,
             inc=inc,
             registration=registration,