GitLab

Removed AVX code, since it had very little effect on performance and would have required a more complicated build process.  Also worked around a compilation error with clang.

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

Integrators control when atom reordering gets done, instead of having it happen automatically when forces are evaluated.  This simplifies some code, and also improves unintuitive behavior with CustomIntegrator.