Experimental implementation of SB2 model

[adrn]

This is a rebase of #107 using the new infrastructure from v1.3.

Right now, this only works if the velocity measurements can be identified with the correct binary component. Ideally, the implementation would not add new classes, but this is just to show that it is possible :)

Demo to be updated! ~~A small demo here: https://gist.github.com/bf2ed66f33e65326ca511fd78754e097~~