netcdf compatibility with gdal

[NPounder]

Pulling this out from #2 as a separate issue.

There is currently some issue with reading the netcdf files with gdalinfo. If there are variables in the root group then gdalinfo can read the file however it does not find the variables in the other groups. If there are no variables in the root group it cannot open the file, even if there are variables in the other groups.

Possibly the version of gdal I am using (2.1.0) can't handle groups. I will look into it further.

[jgomezdans]

Don't bother about groups then, I don't think they're needed ;)

[NPounder]

I'm fine with that as a solution! currently the create_variable and related methods have group as a non optional input. If it's not going to break anything elsewhere I can just delete that part...

[NPounder]

While we are breaking backward compatibility ;-) CF conventions recommend (but don't require) that dimensions are ordered t,(z,)y,x but we've got t,x,y. We don't need to change it (although I don't know if some (netcdf) readers may assume the ordering) bit if we want to change it - probably better sooner rather than later - depends how it will fit with the way you handle the arrays I guess. I leave it up to you.

[jgomezdans]

I don't particularly care about following netcdf standards, GDAL is far more useful, as most netcdf readers will struggle with projections anyway. So if it flies with GDAL, good, if not, back to GeoTIFF ;)