Add calculation features to box

[mphoward]

We should add functions to the box so that it supports wrapping and conversion between real and fractional coordinates.

[clpetix]

Additionally, we should add the ability to center a LAMMPS box about zero. We currently throw an error for uncentered boxes in Snapshot.to_hoomd_gsd(), but since LAMMPS has no limitation on box centering, we should try to accommodate that.