Remove flavour_name and simplify the encode_ options

[alexamici]

Now the flavour is just a definition of which encode_ option to set. On the other hand the name encode is wrong and the option are very long.

Probably change to something like:

>>> cfgrib.open_dataset('data.grib', backend_kwargs={'encode_cf': ['parameter', 'time', 'vertical', 'geography']})

Where the strings accepted are the ecCodes namespaces.