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
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
add and update figures for MUSCL notebook (svg included)
add updated figures to MUSCL notebook
Explain special characters _ and ^

And also explain further why symbols are not created automatically in
SymPy.
Explain simplify( )

Also add note about exponentiation with **