Add cmorizer for Wetland Area and Dynamics for Methane Modeling (WAD2M) dataset #4108
Description
Add a downloader and formatter for the Wetland Area and Dynamics for Methane Modeling (WAD2M) dataset (https://apgc.awi.de/dataset/monthly-wad2m-2000-2020) containing the wetlandFrac variable (see table entry at https://github.com/ESMValGroup/ESMValCore/blob/a748547e7b93ef84aaff83eb4031deb2abdda5a1/esmvalcore/cmor/tables/cmip6/Tables/CMIP6_Emon.json#L5885)