Started 1st notebook of 3rd module

    * Added CFDPython's Burgers' lesson to use as a base
typo fix
Modified notebook 1 of module 3

    * Added welcome message
    * Added discussion on conservative schemes
    * Added derivation of traffic model
    * Changed all references to Burgers' equation to traffic model in Lax-F, L-W (only text) and Mac
    * Need to check L-W derivation
    * Having trouble with MacCormack (doesn't update)
    * Haven't touched B-W
cubic flux version
solve for cubic flux equation using sympy
Worked on first notebook, module 3

    * Added discussion on wave speed
    * Added discussion on stability of backward difference
    * Added example with upwind
    * Added discussion on CFL for nonlinear waves
Merge remote-tracking branch 'refs/remotes/origin/wave_dev' into wave_dev
add derivation of cubic traffic flux
rename cubic notebook
Edits on notebooks 1 and 2 of module 3

    * Separated into two notebooks
    * Removed upwind from first notebook
Edits on convection schemes notebook

    * Removed upwind
    * Modified Lax-F and Lax-W to the "red light" example
    * Edited Lax-W derivation to traffic model
    * Edited mistakes in Lax-W derivation
    * Added stencil and square wave figures from PASI lessons
Edits on convection schemes notebook

    * Added discussion on reversed MacCormack
    * Added Dig Deeper section:
        * Explains why we introduce new methods
        * Discusses shock velocity
    * Changed references to CFL to \Delta t/\Delta x
finish calculation of new flux function
Merge branch 'wave_dev' of https://github.com/numerical-mooc/numerical-mooc into wave_dev
first pass language edits
add pipe flow figure
add ftbs with cubic model
Full stops and other trivialities.

Clearing output has got rid of the JS as well, but there's no change to any code.
Full stops and similar again.
finish up explanation of sympy derivation of flux model and show new results