bumping version to 0.2.21.dev for development
docstring updates
removing unused custom exceptions from _nanotubes module
removing unused custom exception classes from nanogen modules
adding new module for generating fullerene bravais lattice structures
fix module import
cleanup whitespace and other minor changes
docstring updates
whitespace cleanup
adding new modules for json and zmatrix format structure data
updating docstrings with chirality parameters
raise TypeError for bad arguments passed to VacancyGenerator constructors
adding new atoms sub-package to structure_io
removing unused theme files
renaming theme bootstrap files
change references to spc to sknano
minor doc updates
Merge branch 'new_atom_dev' into dev
updating doc sources
updating module imports