Order

- Fixed API.
- Cleaned copy interface.
- Documentation.

Added arguments to all constructors to initialize values stored in an `UniqueObjectIndex`. Values of tuples or dicts are expanded. Example:

*Before*:

python
dataset = order.Dataset("ttbar", 1)

Updated to BSD-3-Clause license.

- Strictly follow linting rules
- Removed unused imports
- Fix some typos.