Add warning for not Number type (debug mode)

Line 154 in 212568d

GeoJSONVT.prototype.getTile = function (z, x, y) {

If x, y, z are string type, getTile returns null. Wasted a lot of time to find it out 🙄

Or at least please add it to README.
Thanks.

oh boy, I too spent a lot of time scratching my head wondering why this wasn't working. For the most part JS will naturally convert to a number for most of the code when doing arithmetic, except at

geojson-vt/src/index.js

Lines 194 to 196 in aa2522b

    
           function toID(z, x, y) { 
        
               return (((1 << z) * y + x) * 32) + z; 
        
           }

It decides to concatenate numbers first, so the results are completely messed up.

I've proposed #128 as a fix, I'd rather getTile just fix up any numbers passed as strings.

👍(personally, I prefer more obviously conversion with Number() 🙄)

	function toID(z, x, y) {
	return (((1 << z) * y + x) * 32) + z;
	}