Description of the desired feature

The GMT side is working on the lunar relief dataset for remote use and it's close to being finished (GenericMappingTools/gmtserver-admin#167). Similar to other load_earth_* functions (xref #1786), we need to add a load_moon_relief function to download and load the data.

• Moon relief dataset Add load_moon_relief to load Moon relief dataset in various resolutions and registrations #2674
• Mars relief dataset Add load_mars_relief to load Mars relief dataset in various resolutions and registrations #2847
• Mercury relief dataset Add load_mercury_relief to load Mercury relief dataset in various resolutions and registrations #3028
• Pluto relief dataset Add load_pluto_relief to load Pluto relief dataset in various resolutions and registrations #3027
• Venus relief dataset Add load_venus_relief to load Venus relief dataset in various resolutions #2906
• GSHHG Earth Distance to Shoreline Remote datasets: Add "load_earth_dist" to load "GSHHG Earth distance to shoreline" dataset #3706
• IGPP Earth East-West Deflection and IGPP Earth North-South Deflection Remote datasets: Add load_earth_deflection to load "IGPP Earth east-west and north-south deflection" datasets #3728
• IGPP Earth Free-Air Anomaly Uncertainty to pygmt.datasets.load_earth_free_air_anomaly Remote datasets: Add "uncertainty" parameter to "load_earth_free_air_anomaly" to load the "free-air anomaly uncertainty" dataset  #3727
• CNES Earth Mean Sea Surface Remote datasets: Add "load_earth_mean_sea_surface" to load "CNES Earth Mean Sea Surface" dataset #3717
• CNES Earth Mean Dynamic Topography Remote datasets: Add "load_earth_mean_dynamic_topography" to load "CNES Earth Mean Dynamic Topography" dataset #3718