Refactor Kokkos functors to 3D MDRange, remove SIMD specializations, and update Field view

[Amazingkivas]

Motivation

• Simplify and generalize the Kokkos functors to iterate over full 3D domains with a consistent index ordering and eliminate complex SIMD-specific code paths.
• Improve memory semantics for field arrays by using the appropriate Kokkos::View memory traits and the project FP typedef.

Description

• Changed ComputeE_FieldFunctor and ComputeB_FieldFunctor to use a 3D Kokkos::MDRangePolicy and updated their operator() signatures from (const int& k, const int& j) to (const int& k, const int& j, const int& i) with flattened 3D indexing.
• Removed all SIMD-specific members and loops (including simd_type, simd_width, start_i/end_i, and Kokkos_SIMD.hpp usage) and replaced with scalar per-index updates and simpler periodic-boundary handling.
• Updated kokkos_shared.h to remove Kokkos_SIMD.hpp, switch Field to Kokkos::View<FP*, Kokkos::MemoryTraits<Kokkos::Restrict | Kokkos::Aligned>>, and use the project FP type consistently.
• Fixed a small signature typo for coef_dz (ensuring it is passed by reference) and simplified index computations for neighbor accesses (i_pred_idx, j_pred_idx, k_pred_idx / scalar_*_idx).

Testing

• Built the project with the updated headers and Kokkos integration using the usual build system; the build completed successfully.
• Ran the repository's automated unit tests and the FDTD example/run tests; all tests passed.

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Codex Task