Accessing submodules

Is there any reason not to allow access to Python submodules from the `orbit` module?

```python
import orbit

bunch = orbit.core.Bunch()
lattice = orbit.teapot.TEAPOT_Lattice()
...
```

Currently we have to write:

```python
from orbit.core.bunch import Bunch
from orbit.teapot import TEAPOT_Lattice
...

bunch = orbit.core.Bunch()
lattice = orbit.teapot.TEAPOT_Lattice()
...
```