fix examples

@lileicc 

Work in progress. I fixed many examples, but the following are still broken:

- example/pcfg/simple-pcfg.blog
- example/and-or/scene_face_bkp.blog
- example/and-or/scene_face_integer.blog
- example/and-or/scene_face_real.blog
- example/and-or/scene_face.blog
- example/and-or/or-node.blog
- example/and-or/scene.blog
- example/and-or/scene_face_integer_testcase.blog
- example/and-or/projection3.blog
- example/and-or/scene2.blog
- example/and-or/projection.blog
- example/file-io.blog (false alarm; works fine if started from the correct dir)
- example/background/adaptive-background-mixture.dblog
- example/aircraft-wandering.dblog

I also changed MultivarGaussian, Dirichlet, Multinomial, Discrete to operate on column vectors.

We can either review / merge this now, or fix the remaining examples, then merge.