function getNamespacePrefixForLayer(sLayer) {
		var sPrefix = layer_prefixes[sLayer];
		if (sPrefix === undefined) {
			throw new Error(`Layer ${sLayer} not supported.`);
		}
		return sPrefix;
	}
Returns the namespace prefix to be used for the given layer.
Null means no prefix must be used.
@param {string} sLayer - The target layer
@returns {string|null} The prefix to be used. null means there must not be a prefix