Improve the shape of a chart bundle in the overview

[aAXEe]

Currently the bounding box from each chart is used as rectangle in the generated overview.json to show the coverage of this bundle. This is a rough estimation.

As the metadata contain the shapes of all contained tiles the coverage polygon should be generated from the tiles eg. at zoom level 15.

For implmentation change the handling here:

chart-bundles-compressor/metafileExtractor.js

Lines 56 to 69 in c0dce08

	const n = bundle.bbox[3]
	const s = bundle.bbox[1]
	const e = bundle.bbox[0]
	const w = bundle.bbox[2]
	const featureOverview = {
	type: 'Feature',
	geometry: {
	type: 'Polygon',
	coordinates: [[
	[e, n], [w, n], [w, s], [e, s], [e, n]
	]]
	},
	properties: properties
	}