Add load_mars_relief to load Mars relief dataset in various resolutions and registrations

[seisman]

Description of proposed changes

import pygmt
from pygmt.datasets import load_mars_relief
grid = load_mars_relief(resolution="15m")
 
fig = pygmt.Figure()
fig.grdimage(grid, cmap="@mars_relief.cpt")
fig.show()

Preview of docs: https://pygmt-dev--2847.org.readthedocs.build/en/2847/api/generated/pygmt.datasets.load_mars_relief.html

Addresses #2431