[Lesson 1] Added the solution.
[Lesson 2] Added solution to the notebook.
[Lesson 3] Added solution to the notebook.
Add new first notebook

We were starting with a notebook on the Pendulum, with no explanation
of the problem this module is about.
Tweak the style

Style for h6 now used in copyright notice.
Add mention of Helios, start physics of phugoids

In new Lesson 1.
Tweak style, h2
Add sketch of forces on a glider
Add formulas for forces, Lanchester "Aerodonetics"
Add formulation of no-drag case, with sketch
Add extra bit on zero-drag case
finish derivation of lancaster phugoid motion
remove import * in general and remove math library from lesson 1
recreate missing mathjax shortcut for \dt = \Delta t
replace np with numpy, remove universal math import
remove numpy as np, remove universal math imports
fix long-standing typo in title
remove math import entirely as they weren't being used
Delete unused style sheet
rename phugoid to match lesson order convention