adjust zoom level dependent on number of tiles 

@dnomadb @yhahn here is where I am headed with adjusting the zoom level dependent on number of tiles requested. Some questions:

* a request like [this](https://api-elevation-surface.mbxsandbox.com/mapbox.mapbox-terrain-v1/?layer=contour&field=ele&zoom=11&access_token=pk.eyJ1IjoiYm9iYnlzdWQiLCJhIjoieVJjWnhZVSJ9.N3FwPuN7VZZLUJSzYKXkjA&encoded_polyline=_d{rEd}upRXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaDXpaD) loads 267 tiles but to me seems like a legitimate request to make against the api. how are we going to determine `threshold`
* should threshold be a ratio? number of points per tile or just number of tiles?

Still need to add tests, benchmark