Fix the code to use the correct formulae.

* Use the sign convention and formulae from Milne-Thomson.
* Error was due to ignoring the fact that the z-axis pointed downwards.
* Compute the normal and the rotation using the right sign convention.
* Change the symbol for height from y to z.
* Update all the comments.
Change pi/2 to -pi/2 for the final flight path.
Use 0 as subscript for the initial values.
Merge pull request #22 from anushkrish/master

Fix phugoid.py to use the correct formulae
add dummy text to base of README
Remove forgotten dummy text
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?