Add metadata to Earth relief grids

[leouieda]

Description of proposed changes

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

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.