Seysen basis reduction (of real and reciprocal lattices simultaneously)

[brainandforce]

For a number of operations, it'll be useful to transform the lattice vectors of a crystal basis to the smallest possible set of vectors - useful for measuring distances between atoms across unit cells. In general, I expect this won't be a big issue for unit cells, since a lot of them already consist of the shortest possible basis, but it can be useful for getting the correct metrics if that isn't the case.