add either convenience function or object within System class that returns ra,dec dist for stationary, median tracks

[wbalmer]

seems like this would be useful for users who want the curve, but (for whatever reason) don't like our plots ;)

[wbalmer]

not necessary - I just added a section to the tutorial that explains how to access System.radecdists and generate your own tracks for arbitrary data.

this does however make me wonder whether radecdists should be moved outside of the System class, since it technically doesn't require any of the System attributes to be run.

[wbalmer]

the above idea is now addressed in #52 :)