Allow passing the box vectors in CFConvNeighbors

[RaulPPelaez]

The underlying CFConv implementation supports PBC, but CFConvNeighbors just hardcodes the box vectors as null.

NNPOps/src/pytorch/CFConvNeighbors.cpp

Line 74 in d15cb91

impl->build(positions.data_ptr<float>(), nullptr);

@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.