MLNCluster class interface

A protocol that feature subclasses (i.e. those already conforming to the MLNFeature protocol) conform to if they represent clusters.

Currently the only class that conforms to MLNCluster is MLNPointFeatureCluster (a subclass of MLNPointFeatureCluster).

To check if a feature is a cluster, check conformity to MLNCluster, for example:

let shape = try! MLNShape(data: clusterShapeData, encoding: String.Encoding.utf8.rawValue)

guard let pointFeature = shape as? MLNPointFeature else {
throw ExampleError.unexpectedFeatureType
}

// Check for cluster conformance
guard let cluster = pointFeature as? MLNCluster else {
throw ExampleError.featureIsNotACluster
}

// Currently the only supported class that conforms to ``MLNCluster`` is
// ``MLNPointFeatureCluster``
guard cluster is MLNPointFeatureCluster else {
throw ExampleError.unexpectedFeatureType
}