Add directionality information to Lattice

[mofeing]

A lattice, unlike a topology, has a partial ordering defined into it which we can use for defining directionality; i.e. which site is to the left/right/up/down/parent/child direction of another site. When going to higher dimensions, hardcoding a "left" and a "right" doesn't scale. For that, adding this partial ordering to Lattice would allow us to add also an ordering of the dimensions and from that, name the different dimensions like "left/right", "up/down", "front/back", "parent/child" (on a tree), ...