device debug print helper

[bd4]

With the new default backend (not using thrust), it's not easy to print a single value or small range of values out of a device gtensor object.

Possible APIs:

• member function, .host_view(...slice..), normal view if is host accessible, otherwise copies
• free function, gt::device_print(a, ...slice...)

[bd4]

one advantage of host_view type API, is it could be used in tests to avoid an explicit host vector declaration and copy.