match code to lapply_nhx_surface_emis name and description

[klindsay28]

Pull request #162 changed the name and description of the setting related to nhx surface emissions. The new settings name is lcompute_nhx_surface_emis and the description is
"control if NHx emissions are computed".

However, the flag doesn't control if the emissions are computed.
It still controls if NHx emissions are subtracted from stf(:,nh4_ind).
The code should be changed to match the setting's name and description.
If the flag is .false., then the emissions should not be computed (or subtracted from stf).

[mnlevy1981]

closed in marbl0.28.6