Efficient off-resonance field calculations

Various options based on distance from the point of interest:

• Very distant from non-repeating obstruction group: magnetic dipole equation (1/r^3) based on total susceptibility
• Within/close to any obstruction group: Fourier transform solution pre-computed on grid (obsidian://advanced-uri?vault=vault&uid=b8871ab5-dd53-482c-a4b4-39d26bcdda17)
• Obstructions within 2 grid elements from position are excluded from the Fourier transform. Instead they are computed individually:
  • If still distant compared with size of object: magnetic dipole equation (1/r^3) based on susceptibility of object
  • Otherwise, full equation

Dipole from obsidian://advanced-uri?vault=vault&uid=5abc687a-0181-4f56-84cb-eca292f99bae: \Delta B_z(\vec{r}) = \frac{\chi B_0}{4\pi r^5} (3 z^2 - r^2) = \frac{\chi B_0}{4 \pi r^3} (3 \cos^2 \theta - 1)