…_helio_frame method

Change Summary

Overview

Fixes #3149 - Adds linear interpolation fallback for zero-flux cases in the Compton-Getting correction.

When one or more fluxes are zero, the power-law interpolation in interpolate_map_flux_to_helio_frame produces NaN because log(0) is undefined. This PR detects these conditions and falls back to linear interpolation.

Changes

imap_processing/ena_maps/utils/corrections.py

Modified interpolate_map_flux_to_helio_frame to:

• Detect when either bounding flux is zero or negative (use_linear = (flux_left <= 0) | (flux_right <= 0))
• Compute linear interpolation as a fallback:
  • Flux: flux_left + (flux_right - flux_left) * f
  • Statistical uncertainty: sqrt((1-f)² * σ_left² + f² * σ_right²)
  • Systematic error: sys_err_left + (sys_err_right - sys_err_left) * f
• Use xr.where() to select linear interpolation where needed, power-law otherwise
• Clamp negative flux values to zero (can occur from linear extrapolation)

imap_processing/tests/ena_maps/test_corrections.py

Added 5 new test cases:

• test_linear_fallback_when_flux_left_zero - Verifies correct interpolation when left bounding flux is zero
• test_linear_fallback_when_flux_right_zero - Verifies correct interpolation when right bounding flux is zero
• test_linear_fallback_when_both_fluxes_zero - Verifies result is zero (not NaN) when both bounds are zero
• test_negative_interpolated_flux_clamped_to_zero - Verifies negative fluxes are clamped to zero
• test_linear_and_powerlaw_mixed - Verifies correct method selection in mixed scenarios

Closes: #3149