Added the possibility to read variables in NetCDF-4 groups

[remkos]

Description of proposed changes

This is to introduce the possibility to read variables from netCDF-4 files that contain variables in groups.
For example, a group "ku" with variables "time", "latitude", and "longitude".

This required introducing "," as an optional separator for variables on the command line. Because, otherwise, the "/'" separator for the variables on the command line would be seen as part of the group/variable naming convention.

Example 1: In case of a traditional file without groups (NetCDF-3 or NetCDF-4): file.nc?time/latitude/longitude and file.nc?time,latitude,longitude are equivalent, and will both work.

Example 2: In case we have a group "ku".

• file.nc?ku/time will first check if the group 'ku' exists; if so, then open group 'ku' to find 'time'.
• file.nc?ku/time,ku/latitude,ku/longitude will automatically assume that there is a group 'ku'.

This has been tested with success on both files with and without groups, reading either single or multiple variables, in one group or spread over multiple groups.

Fixes #

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

If approved, I will add examples to documentation and tests.

[PaulWessel]

Hi @remkos, I first thought this had to do with grids but I see this is netcdf tables. I installed the branch and ran the tests and it fails to pass gaps_time.sh which deals with netcdf tables:

gmtmath (gmtio_nc_fopen): NetCDF: No group found. [gtec_tx_daily.nc]
gmtmath [ERROR]: Cannot open file gtec_tx_daily.nc?time/gtec
gmtmath (gmtio_nc_fopen): NetCDF: No group found. [gtec_tx_daily.nc]
gmtmath [ERROR]: Cannot open file gtec_tx_daily.nc?time/gtec
psxy [WARNING]: File <stdin> is empty!

Perhaps you can sort out what goes wrong there since there are no groups I think.
Also, there probably needs to be some updated in the docs.

[remkos]

Indeed. This applies (for the time being) only to tables. I will look into grids later, though I do not expect groups to be that common there.
The above commit should fix the bug. Somehow I did not test it properly.

[PaulWessel]

All tests pass for me, nice. Please add docs and example as you see fit.