checkGroupMaxOpen = function() {
      if (self.config.groupName) {
        self.config.groupName = coerceToArray(self.config.groupName);
        angular.forEach(self.config.groupName, function(group) {
          if (self._$mdPanel._openCountExceedsMaxOpen(group)) {
            self._$mdPanel._closeFirstOpenedPanel(group);
          }
        });
      }
    }
Opens an already created and configured panel. If the panel is already
visible, does nothing.
@returns {!Q.IPromise<!MdPanelRef>} A promise that is resolved when
    the panel is opened and animations finish.