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Without this fix, we see cases in which not all work-items in a thread group end up hitting the same number of barriers, which leads to a hang in OpenCL GPU execution.

* Fixed potential invalid memory access

* Fixed exception

* Fixed range overflow with very large numbers of atoms

* More fixes to overflow with large numbers of atoms

* Fix test failures

Converted the array containing atom block indices for the neighbor list from ushort2 to int.  This removes the hard limit of 2 million atoms.

On Fermi, store forces as 64 bit fixed point and update with atomic operations.  Also various other minor optimizations.