Overlapping shapes

[momchilmm]

Currently it's up to the user to make sure that shapes added to a ShapesLayer are not overlapping. If they are, the permittivity of both those shapes is added in that region, which is probably not expected behavior.

It might be useful to set the permittivity to the one of the last shape, but it's not clear how exactly since the shapes are defined directly through their Fourier components (no real-space grid). Or at least we could raise a warning if something like this is happening.

Also this could be important for optimizations. If overlap can't be handled properly (see above), it could be useful to somehow implement constraints that allow you to enforce that the shapes are not crossing as the optimization progresses.