Adding multipole magnet utilities

[Nathan-Majernik]

Adds functions for synthesizing and analyzing transverse magnetic fields from multipole expansions.

New module: beamphysics.fields.multipole

• synthesize_field — compute B_x, B_y at arbitrary points from a list of normal and skew multipole coefficients
• decompose_field — recover multipole coefficients from azimuthal B_phi measurements on a circle via least-squares fit
• scalar_error — compute normalized RMS field error between two field distributions on a reference circle

Documentation

• Example notebook: docs/examples/fields/multipole_utils.ipynb
• API reference added to docs/api/fields.md

Supersedes #137 (rebased onto current master with updated module path pmd_beamphysics → beamphysics).

[Nathan-Majernik]

#137 (comment)

Done @ken-lauer