Expand or collapse all nodes.
This method does not trigger 'beforeExpand / onExpand' or 'beforeCollapse / onCollapse' callback function.
Please use zTree object to executing the method.
expandFlag = true means: expand all nodes.
expandFlag = false means: collapse all nodes.
return the result of expand or collapse.
true means: expand all nodes
false means: collapse all nodes
null means: have no parent node to expand or collapse.
var treeObj = $.fn.zTree.getZTreeObj("tree");
treeObj.expandAll(true);