New diagnostic for comparison with NOAA Global Monitoring Lab surface flask concentration measurements of trace gases (CH4, CO2, N2O)

[jlenh]

In the spirit of the diagnostic developed for AOD #3048, the idea is to include a diagnostic comparing the surface concentrations of trace gases (notably CO2, CH4 and N2O) to surface flask measurements provided by stations part of NOAA's Global Monitoring Lab.
The trace gas metric compares modelled multi-annual seasonal means (DJF, MAM, JJA, SON) of surface concentrations (co2s, ch4s, n2os) to a climatology developed using ground based observations from NOAA GML. The basis for the data processing and plotting routines is similarly relying on code from #3048.

The CMORized observational data set development will be done under a separate issue.

Branch and pull request
Work ongoing by jlenh (me), current branch state in branch noaa_gml_surface_flask_trace_gases_diagnostic (not pushed yet, problems with access rights as contributor?)