construct pbc_simd less often
We now use SIMD forms of the pbc data structures in multiple places per MD step. It would be slightly more efficient to construct that once per step (e.g. when we know the box has been updated), and would mean we don't have to make temporary SIMD data structures on the stack, for which there is no fully portable implementation in the C++11 standard (support for extended alignment e.g. for SIMD operations is at the discretion of the implementation, which we didn't realise when we used the standard alignas to implement GMX_ALIGNED(), and maybe GMX_ALIGNED() was broken before that, too).