Dilation/Erosion interface in Julia

[lxvm]

This PR adds a dilation_distance keyword to the ProjectionProblem API in the Julia API that provides an easy way to expand the solid region by approximately dilation_distance in all directions (or shrink/erode it if dilation_distance is negative).

Additionally, in the high-level api, the functions ssp1, ssp1_linear, and ssp2 accept an optional trailing positional argument for the dilation_distance.

Tests were added for the adjoints w.r.t. the filtered density in the presence of a non-zero dilation for both the low-level and high-level APIs.

Additionally, I added an example that demonstrates the effect of the dilation/erosion on Cassini ovals.

[stevengj]

cc @smartalecH and @romanodev — it would be good to add this to the Python API too.

[smartalecH]

I have a draft for the python implementation locally -- I'll try to push something soon.

[stevengj]

Note that this is the algorithm from NanoComp/meep#3087 (comment), which I assume is the same as @smartalecH's draft.

Any comments on the API? A dilation_distance keyword (defaulting to 0 or false … in Julia false == 0, negative for erosion) seems straightforward enough.

[romanodev]

The API is straightforward. @smartalecH: if your draft is different than the intended approach, I can take a crack at this (or feel free to address only SSP1, and I will follow with SSP2)