Add short_vec::operator[] for acces to elements

[hkaiser]

This is available from similar libraries (like Vc or Boost.Simd).

[gentryx]

@hkaiser Do you read-only or read-write access?

[hkaiser]

For the implementation of the parallel algorithms we currently just need read-only access. IIUC, the other libraries (Vc, Boost.Simd) provide the operator[] for both, read and write access to the elements.