Add multipole bindings

[jlogan-cfs]

4.0.0 2025-01-16

Added

• Add mlfmm module with limited bindings to Project Rat's multipole methods for B-field and A-field
  • Exposed as cfsem::mlfmm::fields_linear_filament_mlfmm when rat-mlfmm feature is enabled
  • Add vendored dependencies and thin C FFI to support those bindings
  • Available on Windows, linux, and macOS only; disabled on other platforms
• !Remove flux_density_biot_savart backwards-compatibility alias
• !Upgrade flux_density_linear_filament, vector_potential_linear_filament, and body_force_density_linear_filament functions
  • Now handle finite wire length and finite wire thickness analytically
  • New wire_radius input
  • Removed mixed-precision section from flux density calculation in favor of all f64; this no longer affects overall throughput
    • For both linear_filament and point_segment variants
  • Old point-source segment formulations moved to point_source::segment module and available as flux_density_point_segment and vector_potential_point_segment functions
• Add biot_savart.py and vector_potential.py examples with plots comparing linear filament, point segment, and multipole calcs

[jlogan-cfs]

Status: this branch functions locally, but the build of the C bindings is failing on Windows and Linux in deployment CI largely due to the difficulty of generating delocateable builds of Boost. MacOS builds are working both locally and for python extension distributables.