mfukizawa/EISCAT_3D_v_E_inversion: v1.1.1

Changes Electric-field uncertainty propagation now uses the full 3×3 velocity covariance matrix at each grid point via the magnetic-field cross-product operator: Cov(E) = [B]_x Cov(v) [B]_x^T. Hyperparameter estimation now uses an Armijo backtracking line search instead of Wolfe conditions for improved robustness under bound constraints. Code comments were translated from Japanese to English for public release and improved readability.