Allow passing the box vectors in CFConvNeighbors

[RaulPPelaez]

The underlying CFConv implementation supports PBC, but CFConvNeighbors just hardcodes the box vectors as null. https://github.com/openmm/NNPOps/blob/d15cb9196e283b6b55f88a93d85232458f64fa18/src/pytorch/CFConvNeighbors.cpp#L74 @raimis I believe you wrote this, do you remember why it was decided not to include the box vectors here? I am trying to add PBC to TorchMD-Net and stumbled upon this while trying to port the optimized GN there, which uses NNPOps components.

[raimis]

Because I didn't need PBC, so I cut some corners.