PurpleAir layer requests too much data - optimize

[jywarren]

leaflet-environmental-layers/src/purpleLayer.js

Lines 46 to 50 in e971a95

	var PurpleLayer_url = 'https://www.purpleair.com/json?fetchData=true&minimize=true&sensorsActive2=10080&orderby=L';
	if (typeof self._map.spin === 'function') {
	self._map.spin(true);
	}
	$.getJSON(PurpleLayer_url, function(data) {

we should be narrowing by bounding box-- maybe i misunderstand this code but https://www.purpleair.com/json?fetchData=true&minimize=true&sensorsActive2=10080&orderby=L returns a TON of data, vs:

https://www.purpleair.com/data.json?fetchData=true&minimize=true&sensorsActive2=10080&orderby=L&nwlat=45.67548217560647&selat=41.79179268262892&nwlng=-101.03576660156251&selng=-93.93310546875001