Description of the desired feature

GMT now provides earth relief datasets from different data sources:

• earth_relief: global earth relief data from SRTM15+ (resolutions from 01d to 01s)
• earth_synbath: new earth relief data that uses the statistical properties of young seafloor fabric and directions of spreading is also available and provides more realistic relief in areas of young seafloor with small seamounts (resolutions: from 01d to 01s) load_earth_relief: Add the support of data source 'synbath' #2162
• earth-gebco: earth relief data from GEBCO (resolutions: 01d to 01s) load_earth_relief: Add the support of data source "GEBCO" #1818
• earth-gebcosi: earth relief data from GEBCO but gives sub-ice elevations (resolutions: 01d to 01s) load_earth_relief: Add the support of data source 'GEBCOSI' #2192

References:

• https://www.generic-mapping-tools.org/remote-datasets/earth-relief.html
• https://www.generic-mapping-tools.org/remote-datasets/earth-gebco.html

Instead of having more load_earth_xxx functions, I think we can extend the load_earth_relief function by adding a new parameter (for example subtype or data_source), with possible values: relief (default), synbath, gebco, gebcosi.

Other non earth_relief data sources:

• earth_age Add function to import seafloor crustal age dataset #1471
• earth_faa Add load_earth_free_air_anomaly function for Earth free-air anomaly dataset #2238
• earth_geoid Add load_earth_geoid function for Earth Geoid dataset #2236
• earth_vgg Add load_earth_vertical_gravity_gradient function for Earth vertical gravity gradient dataset #2240
• earth_mag Add load_earth_magnetic_anomaly function for Earth magnetic anomaly dataset #2196, earth_mag4km load_earth_magnetic_anomaly: Add mag4km parameter to support earth_mag4km dataset #2239, earth_wdmam Add WDMAM dataset to load_earth_magnetic_anomaly #2241
• earth_mask (pygmt.grdtrack: Remove the warning about the incorrect parameter order of 'points, grid' (warned since v0.7.0) #2312)