Linear SCC fails to simplify planar multibody pendulum


Example https://github.com/JuliaComputing/Multibody.jl/blob/linear_scc_branch/test/test_PlanarMechanics.jl#L55

I get 
```
Unknowns (3): see unknowns(model)
  body₊phi(t): Rotation angle
  body₊w(t): Angular velocity
  rod₊phiˍtt(t): Angle
```
where `rod₊phiˍtt(t)` should not be there