Split atmostpheric products (from cop_ads, cop_cds, ecmwf) by variables and time.

• EOProducts split by time. Use a new plugin parameter to make the time delta configurable (split disabled if empty):

  api: !plugin
    type: CdsApi
    products_split_timedelta:
      # dateutil.relativedelta kwargs
      month: 1

• EOProducts.assets split by variables. Make separate variables available through assets once download assets subset #721 is done. Variables parameter should be identified through a new plugin parameter:

  api: !plugin
    type: EcmwfApi
    assets_split_parameter: param
  # and
  api: !plugin
    type: CdsApi
    assets_split_parameter: variable

Assets split must also be configurable by product type (see CDS product types) to handle various parameters / variables combinations.

Use product types specific constraints to generate appropriate items:

• http://datastore.copernicus-climate.eu/c3s/published-forms/c3sprod/<DATASET_NAME_CDS>/constraints.json
  (e.g. http://datastore.copernicus-climate.eu/c3s/published-forms/c3sprod/derived-near-surface-meteorological-variables/constraints.json)
• https://datastore.copernicus-climate.eu/cams/published-forms/camsprod/<DATASET_NAME_ADS>/constraints.json (e.g. https://datastore.copernicus-climate.eu/cams/published-forms/camsprod/cams-global-ghg-reanalysis-egg4/constraints.json)