Calculate profiles for trajectories

Calculate profile for each frame in a given trajectory. Should leverage the function described by #39. Can be used to generalize the gromacs `gmx sans` and `gmx saxs` tools to other trajectories understood by MDAnalysis.

```py
generate_profiles(atom_group, trajectory, profile_property_class):
    # type: (AtomGroup, Trajectory, <T>) -> List[<T>]
    pass
```