Getting rid of the Metaclass and the 3D class

[ocefpaf]

@ayan-usgs I know that this is a drastic change, but I believe the SGRID conventions should not cover 3D staggered grids. And, if that is revised to do so, we can simplify the code a lot and lower the entry barrier for newcomers.

[ayan-usgs]

@ocefpaf,

That's fine. I was intending to remove it. There aren't any use cases for 3D grids. The only reason it's still there was because I never got a response from @hrajagers whether he was okay with dropping 3D grid from the conventions.

[ocefpaf]

Awesome 😄