…r Euler equations.

* master: (323 commits)
  Fix some typos in Traffic_flow.
  Incorporate suggestions from @maojrs.
  Update Traffic_flow incorporating @rjleveque's latest suggestions.
  Update README and Index.
  Fix broken import of Burgers Riemann solver.
  Add WC blast wave problem to Euler_compare.ipynb.
  Add examples to Burgers_approximate.
  Various revisions.
  Make all plots appear correctly in all formats in the Introduction.
  Require seaborn package.
  Revise approximate solver for Burgers chapter.
  Revise intro to part two.
  Revisions to Acoustics.ipynb.
  Revisions to shallow water chapter.
  Revisions to traffic flow notebook.
  Improvements to Introduction and Advection notebooks.
  Get PDF compiling again with chapters form parts 1-2.
  Remove latex macros from notebooks.
  Address comments of @rjleveque for traffic with an on-ramp.
  Revise approximate SW notebook based on comments from @rjleveque.
  ...