Provide additional models for growth of voids

[WardLT]

Try a model which includes competing effects:

• Larger void -> more surface flux -> faster growth
• Large void -> larger surface energy -> slower growth?

[Need to work out the math on this, after talking more to Wei-Ying]

[WardLT]

Kind of like Gibbs-Thompson coarsening, but without the volumetric term related to for the formation energy of the percipitate