[ClusterStats] compute lazily

[vicb]

As of today the stats are computed eagerly as they are constructed:

js-markerclusterer/src/renderer.ts

Lines 35 to 50 in e8eecf4

	constructor(markers: Marker[], clusters: Cluster[]) {
	this.markers = { sum: markers.length };
	const clusterMarkerCounts = clusters.map((a) => a.count);
	const clusterMarkerSum = clusterMarkerCounts.reduce((a, b) => a + b, 0);
	this.clusters = {
	count: clusters.length,
	markers: {
	mean: clusterMarkerSum / clusters.length,
	sum: clusterMarkerSum,
	min: Math.min(...clusterMarkerCounts),
	max: Math.max(...clusterMarkerCounts),
	},
	};
	}
	}

That's wasted CPU cycles when the stats are not required (i.e. with a custom renderer).

We should compute the stats lazily when one of the markers's property is first accessed to avoid that.

This could be done in a backward compatible way using getters.