1d stencil examples

This pull request contains:

 - Further unifies the 1D example series
 - Adds  a new example 1d_stencil_8
 - Adds a `hpx::lcos::gather` facility
 - Fixes the return type calculation for `hpx::async_continue`

Except for the `hpx::lcos::gather` utility this patch does not impact any documented API.

Still missing is the documentation for `hpx::lcos::gather` and related tests.