eccodes dependency update

[emanuel-schmid]

Changes proposed in this PR:

• optionally change variable names of eccodes 'gust' variable to allow updating to eccodes > 2.27

This PR fixes #678

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[mmyrte]

Hi @emanuel-schmid, I'm not at all in the know about eccodes and the grib-files being read. I avoided adding eccodes to the dependencies in summer 2020 because it's not pure Python, and because I was only reading minimal amounts of bufr files. I suppose that @ThomasRoosli is much closer to the subject at hand...

[emanuel-schmid]

@mmyrte 👍 fair enough. thanks for the note!