Change topdust into stormdust as it leaves the column

[abierjon]

With revision r2628, top flows scheme is now activated only in 19 columns which contain mountains. Topdust is detrained into background dust as soon as it leaves the column it originated from, since the column it arrives in has a subgrid fraction of 0.
This could be improved by transforming the topdust into stormdust instead when its leaves its column, as the rocket dust storm scheme has an adaptative mesh fraction that only depends on the concentrated dust quantity.