Cfgrib

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- Fix an issue in the README format.

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- Fixed (for real) MULTI-FIELD messages,
See: `45 <https://github.com/ecmwf/cfgrib/issues/45>`_.
- Added a protocol version to the index file. Old ``*.idx`` files must be removed.

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- Fix an important typo in the README. See: `64 <https://github.com/ecmwf/cfgrib/issues/64>`_.

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- Add support for *Windows* by installing *ecCodes* via *conda*.
See: `7 <https://github.com/ecmwf/cfgrib/issues/7>`_.
- Added *conda-forge* package.
See: `5 <https://github.com/ecmwf/cfgrib/issues/5>`_.

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- Fixed a serious bug in the computation of the suggested ``filter_by_keys`` for non-cubic
GRIB files. As a result ``cfgrib.xarray_store.open_datasets`` was not finding all the
variables in the files.
See: `54 <https://github.com/ecmwf/cfgrib/issues/54>`_.
- Fixed a serious bug in variable naming that could drop or at worse mix the values of variables.
Again see: `54 <https://github.com/ecmwf/cfgrib/issues/54>`_.
- Re-opened `45 <https://github.com/ecmwf/cfgrib/issues/45>`_ as the fix was returning wrong data.
Now we are back to dropping all variable in a MULTI-FIELD except the first.

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- Do not set explicit timezone in ``units`` to avoid crashing some versions of *xarray*.
See: `44 <https://github.com/ecmwf/cfgrib/issues/44>`_.