Added summary methods.

Do we assume that all Tracks in a Tracks/TracksCollection object have the same attributes?

```
names(A@tracks[[n]]@data) # is equal for all n?
```
In case we don't, we have to adjust a whole bunch of methods I think (summary, c, ...).

Maybe it makes sense to meet up to shortly dicuss aggregation of trajectories? I'm in today, but can pop in during your office hours tomorrow as well ...