recreate shortcut for dt that keeps getting deleted
added section on Richardson extrapolation in 3rd lesson
Added discussion on Richardson extrapolation
checking a sign, executed style, no changes
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!
Went through first two modules
Added gitignore for files inside lessons
removed pyc file
added corrections
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.
Restored module 01_01
merge suggested changes to phugoid 02
merge changes to phugoid #2 suggested by @Yurlungur