Annoying test output fix

In recent tests, there's some output that claims an error, but doesn't actually throw an error. In particular, you'd get the lines:

```
ERROR : Momentum should not be empty
Think about how to handle this. It should only be None if loaded lazy and in this case it will never be saved.
```

These come from lines 208 and 213 in `storage/snapshot_store.py`. The issue is that we're storing a `Momentum` object with empty velocities and empty kinetic energy. This is an odd thing, and not normally desired; however, in testing (and setup) it can occur. Specifically, it *does* occur in testing as part of the nosetests package-level setup. Our .nc files are still extremely large, and so rather tham include them in the repository, I included a short trajectory in PDB format for test purposes. This trajectory doesn't have velocities associated with it, so it really shouldn't have a `Momentum` object at all. But until now we initialized a `Snapshot` with a `Momentum` object, but that object didn't contain anything.

This PR changes the way we initialize `Snapshot`s so that they only have `Configuration` or `Momentum` objects if they have at least one of the elements stored in those. Otherwise the `snapshot.configuration` or `snapshot.momentum` is `None`.

(There may be cleaner ways of redoing `Snapshot` to handle this, but this was a quick fix so I don't see "ERROR" when my tests pass!)