addition of age-of-air parameters to CLaMS plots

[Jens-Uwe Grooß]

For the Stratoclim project, further parameters from the age of air spectrum have been requested to be displayed: "median_of_age_of_air_spectrum" "fraction_below_6months_of_age_of_air_spectrum" "fraction_above_24months_of_age_of_air_spectrum" They should be added.

[Reimar Bauer]

Addition of the following standard names to the set of CLaMS parameters "surface_origin_tracer_from_north_africa", "median_of_age_of_air_spectrum", "fraction_below_6months_of_age_of_air_spectrum", "fraction_above_24months_of_age_of_air_spectrum"

fixes issue #65

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