Get projection/crs from GMT_GRID_HEADER

[weiji14]

Description of the desired feature

This is a feature request to add a method for getting the projected coordinate reference system (CRS) from a GMT_GRID (#2398)/GMT_IMAGE (#3338)/GMT_CUBE (#3150) struct when parsing a NetCDF/GeoTIFF file. Motivation from #2235 (comment):

In GMT_IMAGE.to_dataarray(), perhaps we should parse the header->ProjRefPROJ4 and set the correct CRS to the 3-band xarray.DataArray. ...

Yeah, I was thinking of parsing the projection information from the header when you mentioned this ProjRefPROJ4 field at #3128 (comment). But ideally we'll need to handle PROJ4/WKT/EPSG:

pygmt/pygmt/datatypes/header.py

Lines 138 to 143 in f97c3a4

	# Referencing system string in PROJ.4 format
	("ProjRefPROJ4", ctp.c_char_p),
	# Referencing system string in WKT format
	("ProjRefWKT", ctp.c_char_p),
	# Referencing system EPSG code
	("ProjRefEPSG", ctp.c_int),

...

Implementation details

Use pyproj.CRS.from_string to make a CRS from PROJ4/WKT/EPSG. Create a new method/property under _GMT_GRID_HEADER at https://github.com/GenericMappingTools/pygmt/blob/v0.13.0/pygmt/datatypes/header.py#L67

We can also consider exposing a .gmt.proj or .gmt.crs xarray accessor property (xref #499). But note that there is some discussion at geoxarray/geoxarray#21 on whether there should be a common .crs accessor in some xarray library that can be used across rioxarray/geoxarray/odc-geo/etc.

Are you willing to help implement and maintain this feature?

Yes