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Improve the performance of sending the AmoebaVdwLambda to Cuda using pinned host memory; Updated the AmoebaVdwForceProxy to version 3, and added backward compatibility to version 2; updated TestAPIUnits.py to handle the per particle lambda flag

This reverts commit 5ef91d22.

* Improve vec3

* Added a ``__neg__`` operator overload so that -Vec3(1, 2, 3) returns
  the same thing as Vec3(-1, -2, -3)
* Derived Vec3 from namedtuple instead of tuple. This allows you to
  access the 3 elements of the vector by name. i.e., vec.x, vec.y, vec.z

* Make sure we use floating point division all the time.