3d adjoint filters

[stevengj]

It would be good to make the adjoint code dimension-agnostic, i.e. it should work for any Lx × Ly × Lz material grid and do filtering along all of the axes, specializing to 2d or 1d filters when one or two of the dimensions are zeros.

cc @smartalecH

@mochen4, when you were trying out full 3d TO, were you just skipping filtering?

[smartalecH]

I think Mo was using #1625, which is dimension-agnostic.

I started making #2016 dimension-agnostic as well but figured it would be best to implement that in a new PR. Doing so is rather straightforward, luckily.

[mochen4]

I think Mo was using #1625, which is dimension-agnostic.

That's right. I used the filters.py from that branch.

[smartalecH]

(cc @mawc2019 )