Resolve "Fix sublattices no unique symbols bug"

summary of changes

• put back the use of set in sublattices init so we only get unique sublattices
• added more symbols available for sublattices
• Now we throw an understandable error message if the number of sublattices exceed the number of available sublattice symbol
• Sublattice class now has a symbol property to easily access the str representation of the sublattice

Closes #360 (closed)