Merge pull request #6 from SimTk/master

update
Merge pull request #8 from SimTk/master

update
added atom selection example
up
cleared cells
proofread
proofread
clarification
clarification
Update atom-selection.rst
Update atom-selection.rst
Update atom-selection.ipynb
cleared cells
Merge pull request #631 from cxhernandez/atom-sel-docs

Added Atom Selection example