QuasiUniformGrid and min_steps_per_sim_size`

[weiliangjin2021]

Two main features:

• QuasiUniformGrid: grid is roughly uniform, but locally snapped. This makes it more useful than UniformGrid in many simulations.
• In AutoGrid, adds a field min_steps_per_sim_size. Now the grid step size is constrained by both min_steps_per_sim_size and min_steps_per_wvl. In many RF simulations, the entire simulation domain is deep subwavelength. Previously, one needs to set a dramatically large value of min_steps_per_wvl so that the simulation domain has more than 1 grid point. min_steps_per_sim_size will be more useful in those simulations.

[weiliangjin2021]

Uniform grid vs Quasi-uniform grid (from notebook)

[dmarek-flex]

Great thanks @weiliangjin2021, just a couple of small comments.

I also think we should make it clear that snapping_points take priority over override_structure boundaries when they are closer than dl_min. Maybe add a small comment in the snapping_points field of GridSpec.

[QimingFlex]

Thanks @weiliangjin2021 for this great feature! This is useful in CPW simulations when we want to avoid reflections at non-uniform mesh interfaces while still need to snap grid to edges. I have only one slight comment for now.

[momchil-flex]

So in text you describe the QuasiUniformGrid as a type of UniformGrid that still snaps to special points, but in code it seems to be much closer to an AutoGrid (subclassing from the same AbstractAutoGrid now). It makes me wonder about a few things:

• Is the name QuasiUniformGrid appropriate: did you come up with this or is it used somewhere else?
• Should this just be an option in AutoGrid as opposed to its own class? Like, ignore the structure media and use this specific dl, but still apply the auto gridding?
• And anyway if not, maybe at least also explain the connection to AutoGrid, i.e. it uses the same meshing but ignores the media information in favor for a user-defined dl.

Looks great otherwise!

[weiliangjin2021]

• Is the name QuasiUniformGrid appropriate: did you come up with this or is it used somewhere else?

Myself

• Should this just be an option in AutoGrid as opposed to its own class? Like, ignore the structure media and use this specific dl, but still apply the auto gridding?

From user's perspective, it feels much closer to UniformGrid. So originally, I was thinking just adding a flag to UniformGrid to indicate whether to locally tune the grid. But yeah, implementation-wise, it's closer to AutoGrid. So I'm not sure what's the best approach.