Constrained building of lattices

[mattwthompson]

Crystal structures are often build by carving particular regions in or out of a large bulk phase. Due to the poor scaling of this approach, it may be useful to have a sort of lattice.populate_constrained function that builds the lattice up with some constraints. This would be slower than the normal populate function itself but often cheaper approach overall because it would forgo building many points that are to be immediately deleted.