replace non-hidden instance vars with property defs for instance attributes
removing use of less common unicode subscript characters from verbose output
use new method to assign_unique_ids in Atoms class
adding new method to assign unique ID to atoms
refactoring to use Graphene class as superclass to GrapheneGenerator class and several misc updates
fix bug in bundle_density calculation and make more robust to cases where pint module not installed
handle ImportError if pint module not installed
update code to consistently treat the structure_data as an Atoms class object and never a list