Implement phylogenetic models

[SimonCMills]

1. Reduce number of points in example_flocker_data from 100 to 30 (100 is arbitrary, and 30 enables example models to be run quicker while still being enough points to make sensible inference).
2. Fix flock() so that it receives data2 arguments.
3. Added phylogenetic model example to tutorial
4. edited testthat to pass following changes to example_flocker_data()

[jsocolar]

@SimonCMills just checking: I thought that flock could already accept data2 and pass it to brms via .... Was I wrong about this?

[SimonCMills]

Yeah, like you I would have assumed it already passed it. Turns out it didn't though, hence the fix!