Sod's: move reference to the bottom
Sod's test problem, reference
Module 3, lesson 3: missing output and edit ugly phrase
Add shock tube sketch
Edit shock-tube notebook throughout
Final edits, shock-tube notebook
remove muscl notebook for merge into master
Merge branch 'master' into wave_merge

Conflicts:
	lessons/03_wave/03_05_Sods_Shock_Tube.ipynb
Merge branch 'wave_merge'
lesson 3: write intro paragraphs
Add reference to van Leer, 1979
Write text on conservative discretization
Add intro to Godunov method. Still needs work.

The explanation of Godunov's method is confusing and the figure is
wrong. More work to do  here.
Expand out u and f components, for clarity
add richtmyer stencil
Fake line under figure
add richtmyer stencil
Update and embed stencil for Richtmyer
Tweak explanation of Richtmyer scheme
Reorganize text on Godunov. Still needs work.