add sod solution figure and update text of assessment
fix plots to use consistent colors + labels like other lessons, remove unnecessary material
change initial traffic density for ftbs model
add dig deeper challenge to end of lesson
Add intro paragraph

Also, author list in alphabetical order, and modify headings
Add figure 'velocity_and_flux' for lesson 3
Insert figure and edit text on new flux model
Fix notation and improve text on new flux model

changing V to u and making text subscripts \rm
Explain sympy.Eq and edit nearby text
Add little explanation of self.lhs and self.rhs
change an h2 for h3
fix typos -- density distributions are in rho, not u
Merge pull request #78 from numerical-mooc/convection_typos

fix typos -- density distributions are in rho, not u
fix error in LW derivation (sign flip) and change u -> rho in density profile
Merge pull request #79 from numerical-mooc/convection_typos

fix error in LW derivation (sign flip) and change u -> rho in density pr...
finish changing all instances of v to u (my bad)
fix minor issue in mathjax array layout
Fix equation-number references
Merge branch 'wave_dev' of https://github.com/numerical-mooc/numerical-mooc into wave_dev

Conflicts:
	lessons/03_wave/03_03_aBetterModel.ipynb
add first figures for MUSCL notebook with svg source