Move space and field from GPU to CPU

[simonbyrne]

It would be helpful to have a function that can move a space and field from the GPU to CPU (or vice versa), e.g. for plotting.

closed by #1619

Describe the solution you'd like
The question is what the interface should look like:

• todevice(device, field/space)?

Additional context

• we could use a similar approach for switching from MPICommsContext to SingletonCommsContext.

[simonbyrne]

cc @charleskawczynski @sriharshakandala thoughts?

[charleskawczynski]

I like the idea. This should be somewhat straightforward with adapt, right? Are there any challenges you see?

[simonbyrne]

Possibly? I'm trying some things out at the moment