How to use ALPAKA_ACCELERATOR_NAMESPACE in headers?

[makortel]

This issue is a reminder of discussion in #165 on what would be the best way to use ALPAKA_ACCELERATOR_NAMESPACE in headers.

One approach would be something along

namespace ALPAKA_ACCELERATOR_NAMESPACE {
  namespace cms::alpaka_tools {
     ...
  }
}

In the Kokkos version this approach is used for functions that do not take the Kokkos execution space as template argument.

Another approach would be to take the Alpaka Accelerator object via template parameter, but the Accelerator depending on the dimension of the index makes this approach a bit cumbersome, unless we would settle with a common index dimension (#144). In the Kokkos version this approach is used for functions that take Kokkos execution space as template argument (the Kokkos execution space corresponds roughly the Alpaka device + queue + accelerator).

Another approach (taken in #165) would be to use ALPAKA_ACCELERATOR_NAMESPACE explicitly in the function arguments along

namespace cms::alpaka_tools {
  void function(ALPAKA_ACCELERATOR_NAMESPACE::Queue& queue, ...);
}

See original discussion in

• Add AlpakaCore/HistoContainer.h + HistoContainer_t, OneHistoContainer_t and OneToManyAssoc_t tests #165 (comment)
• Add AlpakaCore/HistoContainer.h + HistoContainer_t, OneHistoContainer_t and OneToManyAssoc_t tests #165 (comment)
• Add AlpakaCore/HistoContainer.h + HistoContainer_t, OneHistoContainer_t and OneToManyAssoc_t tests #165 (comment)
• Add AlpakaCore/HistoContainer.h + HistoContainer_t, OneHistoContainer_t and OneToManyAssoc_t tests #165 (comment)
• Add AlpakaCore/HistoContainer.h + HistoContainer_t, OneHistoContainer_t and OneToManyAssoc_t tests #165 (comment)
• Add AlpakaCore/HistoContainer.h + HistoContainer_t, OneHistoContainer_t and OneToManyAssoc_t tests #165 (comment)