Add CMORized NOAA GML surface flask concentration measurement of N2O

[jlenh]

Description

This PR adds the N2O dataset pendant from #3847

• Closes Add CMORized NOAA Global Monitoring Lab surface flask concentration measurements of trace gases (CH4, CO2, N2O) #3839
• Link to documentation:

This work adds a downloader and a formatter to ESMValTool to download and CMORize surface flask measurements of N2O from the Global Monitoring Lab (GML) of NOAA. This work contributes to the ongoing development of a recipe and diagnostic to evaluate model output of surface concentrations of trace gases against the NOAA GML station measurements #3838.

The downloader retrieves event-based and monthly station averages of surface flask measurements of N2O (see NOAA-GML) as an archive. Only the monthly averages of each station are CMORized here and are already computed following the methodology described on NOAA's GML website from the event-based files also present in the downloaded archive. The dataset archives are downloaded separately for each trace gas.

The formatter CMORizes the station monthly averages for a given trace gas by creating a NetCDF file containing all stations for which a location was available in the corresponding event files. The NetCDF file holds a time series for each station with the coordinates station_id, latitude and longitude. The general dataset architecture is taken from #3227 which implemented a CMORizer for time series of surface station observations. This allows the access to all stations data in a single file, speeding up loading times and processing operations. The general formatting functions are implemented in a separate file from which a single function is then imported and called in the respective trace gases formatter files.

Note: the CMORized variable produced here n2os is not available in the CMOR table CMIP6 - Amon. However, in the case of model outputs, it needs to be derived from its 3D counterparts (n2o, see #2611).

Note2: For the N2O dataset and the variable n2os, it requires a new release of ESMValCore to incorporate the addition of the variable in the CMOR tables.

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New or updated data reformatting script

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[jlenh]

Scientific @schlunma or @bettina-gier ? Technical @valeriupredoi ?
If you could review this so it is ready to be merged when I take care of the release 🤓
It is basically the same as for #3847 but for N2O (in the end I did a PR for the diagnostic for all the gases so it did not really change anything to separate the cmorizers for this)