non-singular BIE solver, much faster and effecient interpolation for singular integral, and new magnetic field API

[unalmis]

(see #1360)

• All the solvers are thoroughly tested. See the shared paper to explain this PR. Significantly improved adjoint solve from earlier. It seems like this is still the only differentiable 3d laplace solver on the internet that works on multiply connected geometry.
• Tested and confirmed agreement with BIE3D package.
• This PR includes my FFT and DFT improvements significantly improve free surface optimization (both old and new) and anything that involves singular integrals. Even the flaky GitHub ci benchmarks detect this.
• Non-invasive, backward compatible, new magnetic field api implemented. Without the new api, the code would be unreadable.
• There are other applications besides free surface. Free surface works too.see optimization in shared paper.
• The linear solve to compute the objective function only takes one gmres cycle. Gradient based optimization is very fast. Using a fast multipole technique would greatly improve performance. There are jax libraries that do this, but that can be a later pull request to merge after this.

iterative plots

stop tolerance denotes typical stop tol in most applications. gmres/bicgstab is expected to always invert the linear system in a few iterations. after stateful opt, it will be less.