ERA-NUTS: interim version for 2021, daily data from January 1980 to September 2021

Interim extension for the daily ERA-NUTS dataset.

This is the data used for generating the chart in this tweet: https://twitter.com/matteodefelice/status/1446144816744632320

DISCLAIMER: this version has not been completely quality-checked, the next official version (1980-2021) will be released next January. 

## Data
The time-series have daily frequency and are aggregated following the NUTS  2016 classification. NUTS (Nomenclature of Territorial Units for Statistics) is a European Union standard for referencing the subdivisions of countries (member states, candidate countries and EFTA countries).

This dataset contains NUTS0/1/2 time-series for the following variables obtained from the ERA5 reanalysis data (in brackets the name of the variable on the Copernicus Data Store and its unit measure):

  - t2m: 2-meter temperature (`2m_temperature`, Celsius degrees)
  - ssrd: Surface solar radiation (`surface_solar_radiation_downwards`, Watt per square meter)
  - ro: Runoff (`runoff`, millimeters)

There are also a set of derived variables:
  - ws100: Wind speed at 100 meters (derived by `100m_u_component_of_wind` and `100m_v_component_of_wind`, meters per second)
  - HDD/CDD: Heating/Cooling Degree days (derived by 2-meter temperature the EUROSTAT definition).