Add a default plotting utility before running fits

[sblunt]

It's pretty common for people to mistype astrometry, or otherwise mess up the formatting in some way. This week alone, I talked to three people who had this issue! I think this could be decreased if orbitize! were to by default plot all the data before starting a fit.

[DTCupcakes]

I've added a method in the system class in the new branch to plot data given in ra/dec and sep/pa (converted to ra/dec). I plan to add functionality to separate different objects/planets in the plot by colour.

Would like to know if any of the following functionality would be useful:

• Additional plots for sep/pa/RV
• A way to separate data on the plot by instrument

[sblunt]

Great! Can you make a pull request? I'll try to check it out this week.