Add license to header files

src/pystencils/include/PyStencilsField.h

src/pystencils/include/aesni_rand.h

src/pystencils/include/arm_neon_helpers.h

src/pystencils/include/gpu_defines.h

src/pystencils/include/half_precision.h

src/pystencils/include/myintrin.h

src/pystencils/include/philox_rand.h

src/pystencils/include/ppc_altivec_helpers.h

src/pystencils/include/riscv_v_helpers.h