assert when calling global_ptr<T>::local() on device pointers

[Colin MacLean]

For debug purposes, global_ptr<T>::local() should assert that usage of this function on non-host memory kinds is incorrect usage.

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[Dan Bonachea]

Proposed resolution in PR 340

[Dan Bonachea]

issue #464: global_ptr::local() assert for device pointers rather than returning nullptr

Resolves issue #464.

→ <<cset 500af50b6a8c>>