Cfgrib

--------------------

- Add support for selecting the time coordinates to use as dimensions via ``time_dims``.
Forecasts products may be represented as ``('time', 'step')`` (the default),
``('time', 'valid_time')`` or ``('valid_time', 'step')``.
See: `97 <https://github.com/ecmwf/cfgrib/issues/97>`_.
- Reduce the in-memory footprint of the ``FieldIndex`` and the size of ``.idx`` files.

--------------------

- Add support to read additional keys from the GRIB files via ``read_keys``, they
appear in the variable ``attrs`` and you can ``filter_by_keys`` on them.
This is a general solution for all issues where users know the name of the additional keys
they are interested in.
See: `89 <https://github.com/ecmwf/cfgrib/issues/89>`_ and
`101 <https://github.com/ecmwf/cfgrib/issues/101>`_.

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- Fix a bytes-in-the-place-of-str bug when attempting to write a GRIB on Windows.
See: `91 <https://github.com/ecmwf/cfgrib/issues/91>`_.
- Honor setting ``indexpath`` in ``open_datasets``,
See: `93 <https://github.com/ecmwf/cfgrib/issues/93>`_.

------------------

- Much improved ``cfgrib.open_datasets`` heuristics now reads many more
heterogeneous GRIB files. The function is now a supported API.
See: `63 <https://github.com/ecmwf/cfgrib/issues/63>`_,
`66 <https://github.com/ecmwf/cfgrib/issues/66>`_,
`73 <https://github.com/ecmwf/cfgrib/issues/73>`_ and
`75 <https://github.com/ecmwf/cfgrib/issues/75>`_.
- Fix conda dependencies on Python 2 only package,
See: `78 <https://github.com/ecmwf/cfgrib/issues/78>`_.

---------------------

- Drop support for Python 2, in line with *xarray* 0.12.0.
The 0.9.6.x series will be supported long term for Python 2 users.
See: `69 <https://github.com/ecmwf/cfgrib/issues/69>`_.
- Sync internal ecCodes bindings API to the one in eccodes-python.
See: `81 <https://github.com/ecmwf/cfgrib/issues/81>`_.
- Source code has been formatted with ``black -S -l 99``.
- Added initial support for spectral coordinates.

--------------------

- Improve merging of variables into a dataset.
See: `63 <https://github.com/ecmwf/cfgrib/issues/63>`_.