Separation tag can now have multiple units

[ryanvarley]

Separation no occurs twice with units in both arsec and AU, for now I'm going to make the code default to fetching AU only.

Some possible solutions are

• modifying the quantity class to make this conversion on a unit by unit basis (but this could get mixed with real conversions rather than different measurements
• making a function return the separation with an option rather than the current variable
• separate variable for each (note several measurements per quantity may become the norm)