More sophisticated lattice boundary conditions

Something that is missing from the current lattice definitions like square/grid, triangular, and hexagonal is the concept of orienting those lattices in different ways, for example rotating them by a certain angle, which can be useful for 1D tensor network algorithms like DMRG where it is common practice to perform the same calculation on cylinders built from wrapping the same lattice in different ways to see how the ground state changes or doesn't change.

This may require adding on some concept of translation vectors. A simple approach could be to add positions as metadata on the vertices of the graph representations of the lattices (say using DataGraphs.jl, where the vertex data storing the positions could be the Point type from GeometryBasics.jl). Then, lattice rotations could be geometric operations on those positions, and then lattice wrapping could be done through lattice rotations and then cutting certain edges of the lattice (say by only keeping vertices that have positions that fall within a certain rectangular region of space), and then adding back edges to implement periodic boundary conditions.

@JoeyT1994 @emstoudenmire

Start by surveying the current square/grid, triangular, and hexagonal lattice definitions and their graph representations. Then evaluate the proposed translation vectors or vertex position metadata using DataGraphs.jl and GeometryBasics.jl. Done means lattices can be oriented or rotated and wrapped with periodic boundary conditions for the described tensor-network use case.