Add heatmap visualization mode for markers

Heatmap sample code when using Leaflet library (https://leafletjs.com/):

	this._mapObject = L.map(mapDiv).setView([38.845210, 24.333653], 7);

	L.tileLayer('https://b.tile.openstreetmap.org/{z}/{x}/{y}.png', 
		{
			attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>',
			maxZoom: 18
		}
	).addTo(this._mapObject);

	this._markerData[this._markerData.length] = [lat1, long1, 1];
	...
	this._markerData[this._markerData.length] = [latN, longN, 1];
 
	var heat = L.heatLayer(this._markerData, {radius: 15}).addTo(this._mapObject);

Files needed to make that code work:
https://unpkg.com/leaflet@1.6.0/dist/leaflet.css
https://unpkg.com/leaflet@1.6.0/dist/leaflet.js
leaflet-heat.js (from https://github.com/Leaflet/Leaflet.heat)

Start by locating the marker-rendering entry point in DeepSeeWeb and reviewing how marker data is passed to Leaflet. Use the supplied Leaflet and leaflet-heat sample as the integration reference; done means a heatmap visualization mode can render the marker data with the requested heatmap behavior.