Treat subviews correctly

[aprokop]

Need to make sure things like

irecv(Kokkos::subview(view, std::make_pair(i, j)), ...);

compile and work correctly. Currently it doesn't because of expects an lvalue for 1st argument. It could be as easy as using const ref any time a view is passed instead of a non-const reference.

Need to go through functions and do updates, and add testing.

Would probably also need to add tests for unmanaged views.