Block-P3M for fast MoM calculations

The Block-P³M solver calculates action-at-a-distance convolution sums that arise in electromagnetics and, unlike the plane-wave fast multipole method, it does not suffer from low-frequency or sub-wavelength breakdown. The solution uses 𝒪 (N log N) operations and 𝒪(N) storage for surface as well as volume distributions of sources. It differs from a single-mesh P³M primarily in that sparse meshes are used and the mesh convolution sum is evaluated using fast Fourier transforms on a nested hierarchy of blocks. Block-P³M gives optimal accuracy and is applicable from statics to high-frequency electromagnetics.