adding imports for XYZAtom and XYZAtoms classes
adding new AtomsConverter class to convert Atom object types
change mass to m and adding element property
removing LAMMPSAtomAttributes class and updating call to Atom constructor
minor tweaks
minor tweaks
implementing AtomsConverter class and using new atoms subpackage classes
set structure_atoms attribute to None instead of using chemistry.Atoms class
updating write method to handle fpath arg. removing check for atoms instance
adding fpath kwarg to write method