host_to_device: Add support for when device views are already allocated

[jgfouca]

Use case that is now supported:

  view_t view("view", dim1, dim2);
  ekat::host_to_device( {raw_data.data()}, dim1, dim2*pack_size, std::vector<view_t>{view});

So, view is an already-allocated view, or could also be an unmanaged view that has already been set up. This was previously not supported as h2d insisted on taking a non-const vector of view handles and it created the device views for you.

Motivation

@whannah1 wants to use the API this way.

Testing

I added a test for this in the pack_kokkos unit tests.