Update to d3 v4.

[adilyalcin]

Once d3 v4 branch has gone stable and some bugfixes follow, update Keshif dependency and implementation to take advantage of improvements.

Candidate features:

• Major d3.selection updates. Read the docs in detail.
• Major d3.transition updates that can improve speed and synchronization.
• d3.curveMonotoneX(context) : Smooth line charts that do not extend beyond the chart limits.
• d3 force layout has been updated. Looks like provides more stability, and hopefully some speed improvements.
• d3.geo.X -> d3.geoX.
• d3.scale.X -> d3.scaleX & interface updates.
• d3.svg.area -> d3.area
• "You can also now derive lines from areas. The line shares most of the same accessors, such as line.defined and line.curve, with the area from which it is derived. For example, to render the topline of an area, use area.lineY1; for the baseline, use area.lineY0."
• d3.time.format -> d3.timeFormat & more updates
• path.projection interface seems to be updated. Not sure if it still supports point(x,y) transforms.
• Remove components of d3 that are not used by keshif.
• Function arguments for d3.min, d3.max, ... have been slightly updated
• Easing functions are renamed. (animated transitions).
• d3.histogram :: Keshif does not use this method, but it has been updated to provide a more concise interface. Might be worth considering.
• ... (to be updated)

Link to d3 changes (v4.0) readme: https://github.com/d3/d3/blob/a7caf9375fe14d7235562af541fe9decf499bbfb/CHANGES.md

[adilyalcin]

Check if d3 ticks implementation is improved for internal keshif use. "If you specify a desired number of bins rather than thresholds, d3.histogram now uses d3.ticks to compute nice bin thresholds. In addition to the default Sturges’ formula, D3 now implements the Freedman-Diaconis rule and Scott’s normal reference rule."

[adilyalcin]

Improve node selection by finding the closest node to the pointer instead of exact match:

" As an alternative to a Voronoi SVG overlay, you can now use simulation.find to find the closest node to a pointer. C

[adilyalcin]

Use d3.request rather than ajax request to remove jquery dependency.

[adilyalcin]

Create a minified d3 dependency that only includes the modules used by keshif.

[adilyalcin]

See https://github.com/d3/d3-array for details on binning / threshold algorithms d3.thresholdFreedmanDiaconis d3. thresholdScott d3. thresholdSturges

[adilyalcin]

Done!