Clean up/fix the benchmark scripts

@utkarsh.ayachit @patchett2002

The benchmark (basic.py, manyspheres.py, etc.) scripts have become outdated and seem to be unreliable. They have errors that can't be reproduced with simple pvpython scripts.

These scripts need to be cleaned up and fixed. I can provide more information if needed.

Edited Aug 25, 2020 by Ethan Stam

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