Rigid body dynamics

See #13.

Note that I defined a handful of latex macros in the second cell of the notebook:

```
$$
% vector
\newcommand{\v}[1]{\mathbf{\vec{#1}}}
% unit vector
\newcommand{\u}[1]{\mathbf{\hat{#1}}}
% dot product
\newcommand{\dp}[2]{#1 \cdot #2}
% cross product
\newcommand{\cp}[2]{#1 \times #2}
% rotation matrix
\newcommand{\R}[2]{{}^{#1} R ^{#2}}
% vector derivative
\newcommand{\d}[2]{\frac{{}^#2d#1}{dt}}
% angular velocity
\newcommand{\av}[2]{{}^{#2} \v{\omega}^{#1}}
% angular acceleration
\newcommand{\aa}[2]{{}^{#2} \v{\alpha}^{#1}}
% position
\newcommand{\pos}[2]{\v{r}^{#2/#1}}
% velocity
\newcommand{\vel}[2]{{}^#2 \v{v}^{#1}}
$$
```

The cell is hidden, so you might not have otherwise noticed it. Maybe these macros should be imported, so that other notebooks can use them.

Do not rely on me to find time to work on this more.