Structure.pbc_distance doesn't respect cell PBC settings

Currently, `Structure.pbc_distance` will check all periodic images of a periodic structure, even if it isn't supposed to be periodic in certain directions. (e.g. surface slabs with periodicity in x,y, but not z direction)

Expected behaviour: Use `cell.pbc` information to determine which periodic images to check for a closer periodic image in `Structure.minimum_distance_translation`. 

