Cfgrib

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- Saves one index file per set of ``index_keys`` in a much more robust way.
- Refactor CF-encoding and add the new ``encode_cf`` option to ``backend_kwargs``.
See: `23 <https://github.com/ecmwf/cfgrib/issues/23>`_.
- Refactor error handling and the option to ignore errors (not well documented yet).
See: `13 <https://github.com/ecmwf/cfgrib/issues/13>`_.
- Do not crash on ``gridType`` not fully supported by the installed *ecCodes*
See: `27 <https://github.com/ecmwf/cfgrib/issues/27>`_.
- Several smaller bug fixes and performance improvements.

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- Assorted README fixes, in particular advertise index file support as alpha.

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- Big performance improvement: add alpha support to save to and read from disk
the GRIB index produced by the full-file scan at the first open.
See: `20 <https://github.com/ecmwf/cfgrib/issues/20>`_.

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- Rename coordinate ``air_pressure`` to ``isobaricInhPa`` for consistency
with all other vertical ``level`` coordinates.
See: `25 <https://github.com/ecmwf/cfgrib/issues/25>`_.

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- Fix PyPI description.

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- Change the usage of ``cfgrib.open_dataset`` to allign it with ``xarray.open_dataset``,
in particular ``filter_by_key`` must be added into the ``backend_kwargs`` dictionary.
See: `21 <https://github.com/ecmwf/cfgrib/issues/21>`_.