Tpetra: Make it possible to use a Kokkos::Device or Kokkos execution space in place of Node

[mhoemmen]

@trilinos/tpetra
#57 depends on this.

1. Move Tpetra objects into an inner, hidden namespace.
2. Use C++11 type aliases in the Tpetra namespace, so supplying a Kokkos::Device or Kokkos execution space in place of the Node works

For example (default template parameters omitted for brevity):

namespace Tpetra {
namespace Impl {
template<class S, class LO, class GO, class Node> 
class MultiVector { /* the actual implementation */ };
} // namespace Impl

template<class S, class LO, class GO, class ExecSpace, class MemSpace>
using MultiVector = Impl::MultiVector<S, LO, GO, Kokkos::Compat::KokkosDeviceWrapperNode<ExecSpace, MemSpace> >;

} // namespace Tpetra

[mhoemmen]

This is a "task" because it's well defined and could be done by one person. However, please note specializations of Tpetra::MultiVector in Stokhos (and Sacado too?). Those must also be fixed.

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