Portion

Fixed
- More tests to cover all comparisons.
- Define `__slots__` to lower memory usage, and to speed up attribute access.
- Define `Interval.__rand__` (and other magic methods) to support `Interval` from `AtomicInterval` instead of
having a dedicated piece of code in `AtomicInterval`.
- `__all__` properly defined.

Added
- Meaningful `<=` and `>=` comparisons for intervals.

Added
- `Interval` supports indexing to retrieve the underlying `AtomicInterval` objects.

Added
- Both `AtomicInterval` and `Interval` are fully comparable.
- `singleton(x)` to create a singleton interval [x].
- `empty()` to create an empty interval.
- `Interval.enclosure()` that returns the smallest interval that includes the current one.

Changed
- Interval simplification is in O(n) instead of O(n*m).
- `AtomicInterval` objects in an `Interval` are sorted by lower and upper bounds.

Fixed
- All operations of `AtomicInterval` (except overlaps) accept `Interval`.
- Raise `TypeError` instead of `ValueError` if type is not supported (coherent with `NotImplemented`).

- Initial release on PyPP.