Check the dust opacity is reasonable in GCM6

[abierjon]

Following r3094, one should add a check on tau_pref_gcm in radiative transfer (aeropacity_mod) when tauscaling_mode is not equal to 1 (GCM6 configuration, no rescaling of the dust), and stop with a nice error message if tau_pref_gcm is e.g. > 50

Example of error message that could be put :
"Error: tau_pref_gcm is > 50 while there is no rescaling to the scenario (tauscaling_mode is not equal to 1). Better stop here. If you want to run with non-rescaled dust (GCM6), please be sure to rescale your dust fields in your start.nc using newstart"

[abierjon]

Resolved by r3125