Change GRIB mapping for moisture_content_of_soil_layer to 2-3-20

[BHFock]

The current mapping for the CFName 'moisture_content_of_soil_layer' with the units kg m-2 [1] to the GRIB code 2-0-22 ('Soil moisture' ,[2]) is not correct. There is a mismatch in the units as the GRIB code [2] is defined in kg m-3.

WMO [3, page 23] depreciates the use of GRIB parameter category 0 (vegetation/biomass) for soil moisture and recommends parameter category 3 (soil products) instead.

I suggest to map the CFName 'moisture_content_of_soil_layer' to the GRIB code 2-3-20 ('Column-integrated soil moisture' kg m-2, [4]).

[1] http://cfconventions.org/Data/cf-standard-names/71/build/cf-standard-name-table.html
[2] http://codes.wmo.int/grib2/codeflag/4.2/_2-0-22
[3] https://www.wmo.int/pages/prog/www/WMOCodes/WMO306_vI2/LatestVERSION/WMO306_vI2_GRIB2_CodeFlag_en.pdf
[4] http://codes.wmo.int/grib2/codeflag/4.2/_2-3-20

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[BHFock]

@arulalant You may be interested in this change as it looks like you used the same mapping in https://github.com/NCMRWF/UMRider/blob/master/others/ncmrwfIRIS/_grib_cf_map.py#L143.