Lattice: Generate Default Name / Use actual `None`

[ax3l]

For the lattice element name argument, instead of "None", adding a good (unique) default name (empty string as Python None instead of "None") would be good to avoid that one has to come up with names for quick lattices.

Suggested Solution

A) Either add no name by default and allow empty would work.
Exceptions: BeamMonitor and Marker, which need a name.

B) Create auto-names of element<N> instead of assigning "None" by default.