Standardize dataset information for SEVIRI and FCI L2 products

[strandgren]

The dataset information provided in the reader yaml-files for the SEVIRI and FCI L2 products should be standardized following the CF conventions where possible making use of standard_name and long_name in a consistent manner. For categorical data the CF Flags (flag_values, flag_meanings) shall be added as well.

See documentation here: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#_description_of_the_data

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