Cell and Cell length

[JoshuaAppleby]

element.cell does not work because lattice.cell_length does not work.
cell_length also is not equal across the DLS ring and would be wrong.
Additionally, does not account for straight 9/13 correctly.

[T-Nicholls]

Note to self: most of our PVs contain the cell number, e.g. SR23C-DI-EBPM-07:SA:X is in cell 23, so this should be easy to see if any elements have the wrong cell number. As for incorrect cell lengths, we might have to compare against MML, where I think cells are called sectors?