spoof hash change to recover file following deletion
restore second order methods in dev branch
Add introductory paragraphs to notebook
Add more text on 2nd-order and RK methods
Second order method clean up
Clean up RK section of 2nd Order Methods notebook

    * Separated code following previous notebooks
    * Cleaned up parts of the code
    * Added comments on results for convergence and accuracy
Clean up Leapfrog section of 2nd order notebook

    * Separated code with text
    * Added if statements if glider does not reach ground
Correct typo.
Make python 3 compatible.

Only requires tweaking print statements etc.
Tweak plots.

1) Remove the line indicating the ground, as we never get near it with
these parameters.
2) Make subplots use the same colours and weights for the same methods.
fix typos and word choice, fix fencepost errors
Edit lesson 4
Edit convergence section, lesson 4
Edit multi-step methods section, lesson 4

Leapfrog fails. Why?
Typos.
Fix parameters so leapfrog doesn't break
Final polishing of the notebook, lesson 4

With a final "Chew on this …" section inviting an experiment with
leapfrog that will misbehave.
Merge pull request #47 from numerical-mooc/phugoid_dev

Phugoid dev Lesson 4
Remove unnecessary hyphenation
Fix typo: the → they