Call radiative transfer for topflows only when needed

[abierjon]

The new version of the mountain top dust flows (see r2628) activates the scheme only in 19 grid meshes. Yet, the subroutine topmons still calls callradite that recomputes the radiative transfer in every columns of the GCM. Since the RT represents a huge fraction of the whole model's computational time, narrowing this computation to the only 19 affected grid meshes would be a non-negligible optimization.

However, this would requires to rewrite the whole radiative transfer to compute it truly independently in the columns.