When launching icCube report how can I get the user name

in order to gather current user information you should setup GVI configuration. It could be done with appropriate method:

var options = {
    root: "ic3-report/app/",
    rootLocal: "ic3-report/app-local/",
    rootVersion: "_IC3_ROOT_VERSION_",
    callback: function () {
        $('#intro').remove();
        var ic3reporting = new ic3.Reporting(
                    {
                        noticesLevel: ic3.NoticeLevel.INFO,
                        dsSettings: {
                            url: GVI_URL
                        }
                    });
        ic3reporting.setupGVIConfiguration(function () {
            var userName = ic3reporting.userName();
        })
    }
};

After that user information will be available inside context. See code above for details.

Update

A more robust solution is adding the code to the local files that are not overwritten with a new version of the reporting. You can use ic3bootstrapLocal function in Admin > Common JS configuration.

function ic3bootstrapLocal(options) {  
   var ic3reporting = new ic3.Reporting({
       noticesLevel: ic3.NoticeLevel.INFO,
    });

   ic3reporting.setupGVIConfiguration(function(){
      ic3reporting.userName()
      options.callback && options.callback();     
   });        
}