netCDF: Implement full read/write support for new NetCDF4 types NC_UB…

[jdemaria]

…YTE, NC_USHORT, NC_UINT and NC_STRING for variables (except for NC_STRING) and attributes.

Only available if NETCDF_HAS_NC4 is defined and only if format=NC4 for writing.
NC_STRING type is supported for reading (scalar and arrays) attributes and is used for writing only for array attributes (scalar are still written as NC_CHAR).
If NETCDF_HAS_NC4 is not defined or format!=NC4, NC_STRING array attributes are written as a single NC_CHAR string using the GDAL {v1,v2,...} convention.

[jdemaria]

I checked that this does not break autotests netcdf.py and netcdf_cf.py

[jdemaria]

Concerning the NC_STRING type, I have seen that a new creation option for vector has been added:
<Option name='USE_STRING_IN_NC4' type='boolean' description='Whether to use NetCDF string type for strings in NC4 format. If NO, bidimensional char variable are used' default='YES'/>
Maybe this option could be shared with the raster driver (impacting only attributes) because user may want create NC_STRING or NC_CHAR? so the option could be moved from layer creation options to dataset ones? I don't know if then we should modify all nc_put_att_text() in the raster and vector drivers to be able to switch to nc_put_att_string()... but I think CF still does not support NC_STRING(?)
(http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2014/057153.html)
For the moment I use NC_STRING for writing only for array attributes (scalar are still written as NC_CHAR).

[rouault]

Committed in r33586.

Regarding USE_STRING_IN_NC4 moved as dataset creation options, this is your call. I've added this capability for layer vectors so as to avoid the auto-grow logic of OGR string fields of non-bounded size, which is costly both in file size and speed. But admitedly probably a bit borderline regarding CF.

[floriandeboissieu]

At the moment, when band names are specified as NC_STRING, GDALGetMetadata is returning an empty value in front of NETCDF_DIM_xxx = although the corresponding values are well decoded:

Metadata:
  NETCDF_DIM_band_DEF={10,12}
  NETCDF_DIM_band_VALUES={B2,B3,B4,B5,B6,B7,B8,B8A,B11,B12}
  NETCDF_DIM_EXTRA={band}

Band 1 Block=10980x1 Type=Int16, ColorInterp=Undefined
  NoData Value=-32767
  Metadata:
    add_offset=0
    coordinates=spatial_ref time
    grid_mapping=spatial_ref
    NETCDF_DIM_band=
    NETCDF_VARNAME=bands
    scale_factor=1

Would it be possible to include NC_STRING support within band metadata (i.e. case NC_STRING in CreateBandMetadata I suppose)?
It would be very useful to have such a support for SENTINEL 2 (example above), and would propagate to its support in QGIS.