This example creates a custom layout for the cluster balloon content.
The layout is specified with the 'clusterBalloonContentLayout' option.
for objects can be created using the factory templateLayoutFactory
and text templates.
Text templates generate the HTML content of the layout from data hashes
passed to the layout constructor.
In this example, the data provider is the cluster where the balloon was opened.
The layout enumerates and outputs the 'placemarkId' and 'balloonContentHeader' fields for all the cluster's geo objects using the 'for' construction.