adding new StructureData container class to sknano.io._base module
updating xyz format class to use StructureIO.structure attribute
updating xyz format test module
reverting kwarg `structure` to `atoms` in XYZWriter class
updating docstring
Updating atoms variable to reference structure.atoms attribute.
Updating test module for LAMMPS data format.
adding structure_data attributes to GeneratorMixin
adding StructureData class to _base module namespace
adding StructureData class to autosummary directive
adding atoms attribute property to StructureIO class
updating code to use atoms attribute of StructureIO class
Adding GeneratorBase to autosummary directive
Changing GeneratorMixin to GeneratorBase and adding __init__
Replacing GeneratorMixin with GeneratorBase class
adding atoms setter deleter decorator methods
updating test modules
adding call to superclass __init__
removing POAV classes from _poav_atom module
Changing code to compute POAV attributes.