Add support to write a `xarray.Dataset` to a GRIB file.

Due to the fact that the NetCDF data model is much more free and extensible than the GRIB one, it is not possible to write a generic xarray.Dataset to a GRIB file. The aim for cfgrib is to implement write support for a subset of carefully crafted datasets that fit the GRIB data model.

In particular the only coordinates that we target at the moment are the one returned by opening a GRIB with the cfgrib flavour of cfgrib.open_dataset, namely:

number, time, step, a vertical coordinate (isobaricInhPa, heightAboveGround, surface, etc), and the horizontal coordinates (for example latitude and longitude for a regular_ll grid type).

Note that all passed GRIB_ attributes are used to set keys in the output file, it is left to the user to ensure coherence among them.

Some of the keys are autodetected from the coordinates, namely:

Horizontal coordinates gridTypes:

• regular: regular_ll and regular_gg
• not target: projected: lambert, etc (can be controlled with GRIB_ attributes)
• not target: reduced: reduced_ll and reduced_gg (can be controlled with GRIB_ attributes)

Vertical coordinates typeOfLevel:

• single level: surface, meanSea, etc.
• pressure: isobaricInhPa and isobaricInPa
• other: hybrid

GRIB edition:

• GRIB2
• GRIB1

[iainrussell]

@alexamici - thanks for the very quick response! I can confirm that the code in the master branch works perfectly for this file now, and I plotted the fields with metview to check that they look identical (they do)!

[zxdawn]

Another error:

AttributeError: 'Dataset' object has no attribute 'to_grib'

Update

We should import to_grib using the newest name.

from cfgrib.xarray_to_grib import to_grib
to_grib(ds, 'out.grib')