Add metadata to earth relief grids (#248)

Add more metadata to the loaded earth relief DataArray and fix issues with stale metadata after slicing the DataArray.

Author: leouieda

gmt/datasets/earth_relief.py

@@ -37,6 +37,17 @@ def load_earth_relief(resolution="60m"):
     _is_valid_resolution(resolution)
     fname = which("@earth_relief_{}".format(resolution), download="u")
     grid = xr.open_dataarray(fname)
+    # Add some metadata to the grid
+    grid.name = "elevation"
+    grid.attrs["long_name"] = "elevation relative to the geoid"
+    grid.attrs["units"] = "meters"
+    grid.attrs["vertical_datum"] = "EMG96"
+    grid.attrs["horizontal_datum"] = "WGS84"
+    # Remove the actual range because it get's outdated when indexing the grid, which
+    # causes problems when exporting it to netCDF for usage on the command-line.
+    grid.attrs.pop("actual_range")
+    for coord in grid.coords:
+        grid[coord].attrs.pop("actual_range")
     return grid