FIX(#13, plot): PY2 era BUG when writing plot-diagrams; give sample code in the into

[ankostis]

• MERGED with #28 to parametrize tests.
• FIX(plot): bring plot writing into files up-to-date from PY2 (were not opening in binary mode).
• FIX(plot): do not open file if file-ext unsupported
• FEAT: support render in Jupyter notebooks.
• ENH: Overlay on the graph these elements: -execution-steps & order
  • inputs
  • outputs
  • solution values
  • "optional" needs:
• TEST(plot): add TC for plot writing; needed to retrofit travis to support graphvis & matplotb.
• REFACT: moved plot code & TCs into own files - split Graphviz building from IO functions.
• enh(build): Test also pip extras now.
• doc: Just a minor but useful addition in the intro tutorial about plotting.

Legend

Nodes:

• circle: function
• oval: subgraph function
• inversed-house: given input
• house: asked output
• polygon: given both as input & asked as output (what?)
• square: intermediate data, neither given nor asked.
• red frame: delete-instruction, to free up memory.
• filled: data node has a value in solution, shown in tooltip.
• thick frame: function/data node visited.

Arrows

• solid black arrows: dependencies (target need source, sources provide target)
• dashed black arrows: optional needs
• green-dotted arrows: execution steps labeled in succession

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[codecov-io]

Codecov Report

Merging #19 into master will increase coverage by 11.81%. The diff coverage is 96.47%.

@@             Coverage Diff             @@
##           master      #19       +/-   ##
===========================================
+ Coverage   77.87%   89.69%   +11.81%     
===========================================
  Files           5        6        +1     
  Lines         348      388       +40     
===========================================
+ Hits          271      348       +77     
+ Misses         77       40       -37

Impacted Files	Coverage Δ
graphkit/base.py	79.36% <100%> (ø)	:arrow_up:
graphkit/network.py	87.03% <71.42%> (+16.83%)	:arrow_up:
graphkit/plot.py	98.68% <98.68%> (ø)

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[ankostis]

Rebased on top of ank/fix-networkx_in_py2(#16) to make Travis pass.

[ankostis]

This is now merge-ready:

• Adde Jupyter support
• Revamped & parametrized TCs with pytest (#28).
• Moved plot code out of network module.

[ankostis]

Forced to drop PY3.4 due to pytest not working...i know, this is one step close to become graphkit2.