pick a sane sampling based on input

[kevinkreiser]

this will update the slider based on the input points so that not too many or too few points are sampled. the goldilocks algorithm we can call it

also allow sending locations in the url:

valhalla.github.io/demos/elevation/index.html?sample=[{"lat":47.22,"lon":9.33},{"lat":47.37,"lon":9.44}]#loc=12,47.2762,9.4410

for example

[dnesbitt61]

:shipit (if the JS folks agree)

[dgearhart]

:bear: :bear: :bear: