Running the Generic PCM with zero aerosols

[emillour]

In the current version (r3182) there is a weird behavior in the case where naerkind=0 in the iniaerosol routine (in aerosol_mod): when naerkind=0, then

! For the zero aerosol case, we currently make a dummy co2 aerosol which is zero everywhere.
! (See aeropacity.F90 for how this works). A better solution would be to turn off the 
! aerosol machinery in the no aerosol case, but this would be complicated. LK

  if (ia.eq.0) then  !For the zero aerosol case. 
     ia = 1
     noaero = .true.
     iaero_co2=ia
  endif

which actually dates back to r741 :-)
But at the time the number of scatterers what set at compile time (and had to be >=1). Now that naerkind is set at run-time, this complexity might be removed and corresponding code cleaned up?