Create mouse down events

adds an atom picked event to be sent to the dom element.

This can allow highlighting of selected residue etc from the application. 

This is some example code to show how to access the events

<code>var callback = function(e) {
    e.preventDefault();    
    console.log(" Atom number=" + e.detail.atom.residue().num());
};
viewer._domElement.addEventListener("atomSelected", callback, false);
</code>

I think it would make more sense to call the event atomClicked rather than atomSelected. "selected" seems to imply that the atom is added to a selection of some kind.