Add support of GetClusterLeaves and GetClusterExpansionZoom on Datasource

[arnaudleclerc]

This PR adds two new methods on the Datasource class :

• GetClusterExpansionZoom, to retrieve the zoom level at which a cluster will expand or break
• GetClusterLeaves, to retrieve the features contained on a cluster

[DavidThielen]

For anyone upgrading to this to test - you may get the following when you run:

JSException: Could not find 'azureMapsControl.Datasource.getClusterExpansionZoom' ('getClusterExpansionZoom' was undefined).
Error: Could not find 'azureMapsControl.Datasource.getClusterExpansionZoom' ('getClusterExpansionZoom' was undefined).

If you do, you need to delete all caches (I don't know which mattered, I cleared them all):

• Browser cached files
• Solution .vs folder
• Project bin folder
• Project obj folder

Once you do that, the exception goes away and it finds the function.