Fabrication Constraints

[satadrudas]

Meep uses a fabrication constraint as per this paper: Zhou, M., Lazarov, B. S., Wang, F., & Sigmund, O. (2015). Minimum length scale in topology optimization by geometric constraints. Computer Methods in Applied Mechanics and Engineering, 293, 266-282.. I think it would be great to add function like indicator_void, indicator_solid, constraint_void and constraint_solid (from meep filters ) to have a minimum spacing and minimum linewidth constraints. For reference, here is an example where minimum linewidth and minimum spacing was used.