fix coordinates in ship detector notebooks

[danabauer]

https://github.com/planetlabs/notebooks/blob/master/jupyter-notebooks/ship-detector/01_ship_detector.ipynb

We're outputting pixel coordinates not lat/lng

  "ship_count": 4, 
  "ships": [
    {
      "lat": 557725.0, 
      "lng": 4176230.0, 
      "id": 1
    }, 
    {
      "lat": 558590.0, 
      "lng": 4176110.0, 
      "id": 2
    }, 
    {
      "lat": 559280.0, 
      "lng": 4176025.0, 
      "id": 3
    }, 
    {
      "lat": 558870.0, 
      "lng": 4174930.0, 
      "id": 4
    }
  ]
}

[sarasafavi]

Closed in #15