recreate shortcut for dt that keeps getting deleted
added section on Richardson extrapolation in 3rd lesson
Added discussion on Richardson extrapolation
Add new figure of forces on the glider

With trajectory pointing up, for a positive angle
Rewrote full-model formulation

Without magically eating constants like in the math notes. The factors
have physical significance!
Correct minor typos and grammar points.
Docstrings to numpydoc format.
Update the code to match the equations.

NOTE: should probably pick "real" numbers; currently chosen to match the original, non-dimensional equations.
Make python3 compatible.

This only needs minor changes to print statements including formatting;
should be backwards compatible with 2.7.
Include solutions for all exercises.

A lot of cut and paste so there is unnecessary code in many places.

Is it worth introducing the use of polyfit to measure the slope of the
lines in the loglog plot to measure convergence that way?
move 2nd order methods to separate notebook + small fixes
Create separate notebook for 2nd order methods and add helper function

01_04_helper_functions.py are loaded with %run magic in 2nd order
methods notebook -- it contains all the functions created in notebook 3
so we don't have to copy and re-run all of them in the 4th notebook.
Delete unused 'stability' notebook
Add realistic parameter values; add IH to authors

Also, improve the text in various places, and check that the code works
like it should.
expand explanation of convergence check
add paper airplane challed (wip)
small edits to text
Remove all reference to error in grid convergence

Instead, always use "difference between grids" or something. It is not
good to get idea in people's head that this is an error. Still have a
bug in the code, though.
Still broken when getting 'diffgrid'

I get nan. Bug.
Fix bug in diffgrid.

Actually return the value!