Update demo notebook

Put a more generic version of the APUG demo notebook in the `examples` directory.

* remove my name, the date, and references to APUG
* rename the notebook "features.ipynb" to make it more generic
* change a call to `.get_localarrays()` to `.localarrays()`
* remove the "(coming soon)" note about slicing
* make the notebook Python 3 compatible (using `print_function`)
* remove the "(coming soon)" note on the slicing demo