Add Connerney et al. (1981, 2020) Jupiter magnetodisc current sheet model

[Beforerr]

Summary

This PR adds support for the Connerney et al. (1981, 2020) Jupiter magnetodisc current sheet model, which describes the magnetic field contribution from a washer-shaped azimuthal current sheet near Jupiter's magnetic equator. This is a key external field component for accurate Jupiter magnetosphere modeling.

Key Changes

• New module src/current_sheet.jl: Implements the complete Connerney current sheet model with:

  • CurrentSheetModel struct implementing the InternalFieldModel interface
  • CurrentSheetParameters for configurable model parameters (current intensity, disc geometry, tilt angles, radial current)
  • CurrentSheetEquationType enum supporting three calculation methods:
    • Analytical: Fast Edwards et al. (2001) approximations
    • Integral: Accurate Connerney et al. (1981) numerical integration
    • Hybrid: Combines both methods based on position (default, recommended)

• Coordinate transformations: Implements conversions between SIII spherical coordinates and the tilted current sheet frame to handle the disc's tilt relative to Jupiter's spin axis

• Field calculations:

  • Cylindrical coordinate field computation with azimuthal and radial current contributions
  • Analytical field expressions with helper functions for efficient computation
  • Numerical integration using Bessel function approximations
  • Radial current contribution (CON2020 feature)

• Public API functions:

  • CON2020(): Creates the 2020 model with modern parameters and radial current
  • CAN1981(): Creates the original 1981 model for comparison

• Comprehensive test suite (test/test_con2020.jl):

  • Model loading and parameter validation
  • Field evaluation at various positions
  • Symmetry properties verification
  • Comparison of different equation types
  • Custom parameter handling
  • Integration with JRM33 internal field model
  • Edge case handling (near axis, at disc edges, outside disc)

• Integration: Updated main module to export new model types and convenience functions

Notable Implementation Details

• Uses polynomial approximations for Bessel functions (J₀, J₁) for efficient numerical integration
• Handles singularities near the rotation axis with special case logic
• Supports both with/without radial current for flexibility between model versions
• Tilt angles configurable in degrees for user convenience, converted internally to radians
• Field components properly transformed between coordinate systems to maintain physical accuracy

https://claude.ai/code/session_01832e6uXkGU6WZZL7qnj6pU