Module 1: Phugoid

I believe this incorporates all the suggestions that have been made, and a few more issues I caught.  I also moved the solutions out of the notebooks and into a solutions directory.  They can be pulled into the appropriate places using the `%load` magic.
This isn't strictly necessary.  
If you use 
```Python
thet = sympy.symbols('theta')
```
Sympy will recognize the greek and render it accordingly