FABs' En-route ATFM Delay Chroropleth

Average En-route delay per FAB in 2015.

TODOs:

• [x] automatic thresholding (quantization?). Use 5 classes instead of the 9 from Mike's bl.ock

• [x] proper legend, maybe using d3.legend

• [ ] Investigate adding FAB labels, i.e. in the relevant centroids, and assign automatically a high contrast (compared to area color [depending on delay value]) font color. See this SO post or this blog post. Also see W3C material. The following simple function from the blog could suffice to decide between black or white label:

  ```javascript
  function getContrast50(hexcolor) {
    return (parseInt(hexcolor, 16) > 0xffffff/2) ? 'black':'white';
  }
  ```
  

• [ ] better tooltips, maybe using d3.tip D3-tip has some benefits but:

  - not easy to have tooltip follow mouse, i.e. inside FAB area (see
    [issues #53](https://github.com/Caged/d3-tip/issues/53) for a possible solution)
  - no solution implemented to avoid cutoff at sides (see
    [issue #117](https://github.com/Caged/d3-tip/issues/117) for a possible fix not yet
    integrated in the official library)
  
  An alternative solution is the use of an helper, see
  [Chris Viau's gist](https://bl.ocks.org/biovisualize/2973775)
  or ([my fix](/espinielli/4d17fa15a7a5084e217992f985fba484) of
  [rveciana's one](/rveciana/5181105))
  

forked from espinielli's block: European FABs