Support automatic dust destruction with an iterative approach

[petercamps]

Description
Allow a portion or all of the dust in each location (i.e. in each spatial cell) to be destroyed based on the local radiation field (or its effect on the dust, e.g. the dust temperature). The subgrid recipe will also depend on the material type and the grain size). This must be part of an iterative process to self-consistently calculate the dust contents and the radiation field in each cell.

Context
This feature is particularly relevant for models including harsh radiation sources such as active galactic nuclei (AGNs). It could conceivably also be used in galaxy-wide models to modulate the gas-to-dust ratio depending on the local radiation field (and the local metallicity).

[petercamps]

This feature is included in pull request #79 which was merged into the public master today. The current implementation has just a basic dust destruction recipe; other recipes can be added as required by specific applications.