incorrect metadata for the decompmicc parameter

[rgknox]

See the metadata describing the decompmicc parameter here:

https://github.com/NGEET/fates/blob/master/parameter_files/fates_params_default.cdl#L168-L170

It states the value should be the mean, but this is not true. The value should actually be the maximum amount found over the depth of the soil column, and then the code will apply an exponential attenuation function to it. The maximum from the parameter file will be applied at a hard coded 7cm of depth with a similarly hard-coded attenuation coefficient, which were designed to match previous decomposition profiles provided by @qzhu-lbl in the ELM parameter file.

See here:
https://github.com/NGEET/fates/blob/master/biogeochem/FatesSoilBGCFluxMod.F90#L680-L687

@jenniferholm @qzhu-lbl