WIP Implement support for 3-D grids

[PaulWessel]

Addresses steps listed in #2912 to make GMT ready to transparently handle 3-D data grids, such as needed in grdinterpolate or produced by greens line. There are several step, and we will do one at the time and test to make sure no bad side-effects sneak in.

This initial commute simply extends the GMT_GRID_HEADER with the required parameters.

[joa-quim]

This is going to break Julia

[PaulWessel]

How can we do things without breaking Julia/GMT? We have to be able to improve GMT and not be told no can do, right, so how?

[joa-quim]

Can we have a different header for 3D?

[PaulWessel]

No, since GMT_GRID contains the GMT_GRID_HEADER. I think this is a version management thing. You will need to require 6.0 of users of your current Julia release, not master. So release a Julia4gmt version that requires 6.0 and done with it. Then, master is always a work in progress and may need to be rebuilt when GMT changes. Most users are probably installing 6.0 anyway.

[joa-quim]

Don't know if this affects julia only or not. If yes, I can always add another if version but if it affects others than we should probably not put this in 6.1

[seisman]

Any schedule for the 6.1.0 release?

[PaulWessel]

The 3-D stuff branch can wait for after 6.1. We should just pick a date since otherwise we keep on adding things and delaying. How about April 15?

[joa-quim]

What else it open that we may want to put in 6.1? If minor things we can do it earlier than A15

[PaulWessel]

Not sure. I think we have fixed a bunch of stuff and added new features worthy of a 6.1 release.
I will fix #2967 problems. Does not look like the other issues are critical (?). I should add re 3-D that since we can read 3-D cubes and extract the slices etc. it is a pretty good advance. The need to write 3-D cubes will be nice but not that urgent - hence working in a branch until that converges is fine with me.

[seisman]

I opened #2968 for tracking the upcoming 6.1.0 release. No critical issues but I expect some final decisions about PR #2013 before the release.