Make some plotting tools

[langfzac]

We should make some basic plotting functionality such as plotting TTVs or displaying positions/orbital elements over time.

To get started:

• [ ] Write a function to compute TTVs from a TransitTiming type.
• [ ] Make new plot and scatter methods that dispatch on TransitTiming.
• [ ] Update elements outputting. (Perhaps link to plotting?)

[giordano]

Make new plot and scatter methods that dispatch on TransitTiming.

If you're using Plots.jl, the standard way to do this is to use https://github.com/JuliaPlots/RecipesBase.jl

[langfzac]

If you're using Plots.jl, the standard way to do this is to use https://github.com/JuliaPlots/RecipesBase.jl

Awesome, thanks!