Adding new StructureFormatter base class for future dev.

This class will provide methods for converting `Atom` attributes to
a formatted string that can then be easily manipulated or written
to file.
renaming XYZDATA to XYZData and adding XYZFormat to docstring
updating class names and adding XYZFormat class
adding dictionary containing ATOM record string format
renaming classes starting with LAMMPSDATA to just DATA for consistency
point LAMMPSDATA to DATAData for backwards compatibility
updating setup for release of version 0.2.23