Code for complex cases such as Neumann-2d or 2D darcy

[liu-ziyuan-math]

Dear authors,

Good work! For now the neukernelcorrection3d in BOON_3d.py is still commented in TODOs, and the code for the Experiment 4.2.2 2D WAVE EQUATION is not available yet. Would you provide this portion of code in future updates?

Additionally, but not very importantly, I wonder if the BOON for 2D/2D-single_step is also feasible. Have you tested them over the 2D Darcy problems in FNO paper?

Thanks!