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.
Add figure shock tube & Riemann
Embed figure and add more on Godunov
Add figure many_Riemann_problems
Explain Riemann problems

plus some small fixes elsewhere
Add references

Godunov (1959) and van Leer et al. (1987)
Explain approximate flux formula

Almost happy now with section on Godunov method. There is some mystery
left on how the flux formula comes about.
Tiny typo.
tweak language, update formatting, pythonify functions
update docstring, add function import