Tab completion for lattice elements

[mattsignorelli]

It would be great if tab completion in the REPL is implemented for lattice elements, so if I do lat.branch[1].ele[1]., and then Tab, it shows all the fields. This could be done by overloading propertynames (see here)

We also should consider implementing Ele <: AbstractDict for all elements to gain access to some features, also shown in that discussion linked above (e.g. iterating, etc)

[DavidSagan]

@mattsignorelli This would be a good project for Lixing or some other undergraduate since it is very self contained. I will mark this as such. If you can think of other projects please post.