Feature request: support for triclinic boxes.

[bdice]

I would like to use the "Diffractometer" code in GIXStapose with triclinic boxes, not just orthorhombic boxes. This probably requires careful handling of periodic boundaries, gridding in non-square systems, and linear algebra for various projections and transformations. The freud library may be useful for this. I would ultimately like to consider including this diffraction code as a module in freud.