Implementing _copy_from_and_resize for OpenCL backend

I got (not so) recent reports about missing operator _copy_from_and_resize… From the signature it seems exactly the same as _copy_from but without non-blocking option.

Can you describe why there are two operators and what is the difference. I currently call _copy_from from _copy_from_and_resize since copy_from allows resizing as well. But not sure what I miss.