Let GMT have ability to write a 3-D netCDF grid

[PaulWessel]

Description of the desired feature

We have two modules (greenspline, grdinterpolate) that need the ability to write out a 3-D grid. Currently, they are only able to write a stack of 2-D grids but this is not a good longterm solution. I intend to implement the 3-D grid writing by leveraging code that @joa-quim has written for some non-GMT programs of his:

https://github.com/joa-quim/NSWING/blob/master/nswing.c#L3050

and

https://github.com/joa-quim/NSWING/blob/master/nswing.c#L3294

This request is mostly to serve as placeholder for this links since my ability to loose emails in the early onslaught of emails is pretty good...

GMT mostly reads 2-D grids and can read a 2-D slice from higher dimension netCDF grids. Thus, there is no immediate need for a function to read in a 3-D grid per se, but this could change in the future. Writing the 3-D grid is a much higher priority and should be implemented before 6.2.0 is released later this year.

[seisman]

Related discussions at #2912.

[PaulWessel]

Closed as implemented by #4581.