MiniAOD for InputGenJetsParticleSelector

This PR adapts the InputGenJetsParticleSelector so it also works with miniAOD. The main difference between AOD and miniAOD is that in miniAOD a subset of GenParticles is stored in the following two collections:

* `prunedGenParticles`
* `packedGenParticles`

and InputGenJetsParticleSelector needs to access both collections in order to perform the same selection of GenParticles as in AOD where all GenParticles are stored in one collection, `genParticles`. Attached are a few comparison plots. Small differences between AOD and miniAOD are expected due to the loss of precision in the GenParticle (un)packing process.

![genjetscomparison_1](https://cloud.githubusercontent.com/assets/4885289/4550513/562a2bd6-4e67-11e4-8891-3c4f62e97f3c.png)
![genjetscomparison_2](https://cloud.githubusercontent.com/assets/4885289/4550515/562f9b52-4e67-11e4-9c15-ccfbeaadfa75.png)
![genjetscomparison_3](https://cloud.githubusercontent.com/assets/4885289/4550512/5629c13c-4e67-11e4-801f-34b36c14573b.png)
![genjetscomparison_4](https://cloud.githubusercontent.com/assets/4885289/4550514/562cb7c0-4e67-11e4-813a-134129c6ba68.png)
In the standard use case (so ignoring the partonic mode) only stable (`status=1`) particles are used in GenJet clustering. In AOD all GenParticles are stores in a single collection, `genParticles`, while in miniAOD there are two collections, `prunedGenParticles` and `packedGenParticles`. The former one stores a small subset of interesting GenParticles while the latter one contains all `status=1` GenParticles (in the packed format) that can be used for GenJet clustering. The packed GenParticles also contain reference to the first available ancestor in the collection of pruned GenParticles. So in order to navigate through the event history, in miniAOD it is necessary to use both collections, `prunedGenParticles` and `packedGenParticles`. However, the collection of pruned GenParticles can also contains `status=1` particles so this particular line ensures there is no double counting of stable particles.