Low-resolution Earth relief grids should support the "region" argument

[seisman]

Description of the desired feature

In load_earth_relief, the region argument only works for high-solution earth relief grids (>=05m). For low-resolution girds, it raises an error:

from pygmt.datasets import load_earth_relief
grid = load_earth_relief(resolution="01d", region=[0, 30, 0, 30])

Error:

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-2-97790979a447> in <module>
----> 1 grid = load_earth_relief(resolution="01d", region=[0, 30, 0, 30])

~/.miniconda/lib/python3.8/site-packages/pygmt/helpers/decorators.py in new_module(*args, **kwargs)
    418                         kwargs[arg] = separators[fmt].join(f"{item}" for item in value)
    419             # Execute the original function and return its output
--> 420             return module_func(*args, **kwargs)
    421 
    422         return new_module

~/.miniconda/lib/python3.8/site-packages/pygmt/datasets/earth_relief.py in load_earth_relief(resolution, region, registration)
     87     if resolution in non_tiled_resolutions:
     88         if region is not None:
---> 89             raise NotImplementedError(
     90                 f"'region' is not supported for Earth relief resolution '{resolution}'"
     91             )

NotImplementedError: 'region' is not supported for Earth relief resolution '01d'

I don't remember why it was not implemented, possibly due to some technical difficulties (related to #542, #524).

Are you willing to help implement and maintain this feature? Yes, but contributions are welcome.