Fix the grid accessor (grid registration and type) for 3D grids

[seisman]

Description of proposed changes

As mentioned in GenericMappingTools/gmt#6588 (comment), the grdinfo output for a 3D grid was wrong in old GMT versions.

After the upstream fix, the grdinfo output is:

$ gmt grdinfo -Cn eraint_uvz.nc     
grdinfo [WARNING]: No 3-D array in file eraint_uvz.nc.  Selecting first 3-D slice in the 4-D array z
-180	179.25	-90	90	200	850	66825.5	66825.5	0.75	0.75	0	480	241	3	0	1

which has 16 columns, instead of 12 columns for 2D grids. Thus, we cannot get the grid registration and type information from columes 11 and 12. Instead, we should use the last two columns instead.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[weiji14]

Looks great, and it's compatible with GMT 6.3 and 6.4!