Trajectory visualization

[zsunberg]

From #65:

My idea was to implement some plotting, save, load... methods on that type. However, I think that could be accomplished on the Vector also...

It would be cool to pop up a plot of a trajectory whenever a ResultsStore is returned.

However, using ResultsStore as the primary mechanism for visualization has a few problems:

1. If someone else wants to use your fancy plotting code, they have to understand and create everything in ResultsStore, like Aircraft, etc.
2. Visualization typically involves many visualization-specific configuration options, for example velocity_vector_color="blue". It doesn't seem appropriate to store these options in ResultsStore.

In my opinion, a better pattern is to have a visualize_trajectory (or similar) function that takes in a trajectory along with visualization options and returns a displayable object. We can talk about more details if you decide to pursue this pattern.

[zsunberg]

(Also, if you do end up having a ResultsStore object, you can implement automatic plotting with something like show(io::IO, mime::MIME, rs::ResultsStore) = show(io, mime, visualize_trajectory(rs)).