Add class to `package` object

[peterdesmet]

To better recognize a package object, e.g. for pretty print (#154), it would be good to extend the list class with a custom class datapackage.

• Add class datapackage
• Add the class with create_package()
• Allow create_package() to start from an existing list and use it in read_package()
• Add tests for create_package()
  • Including for existing lists
• Test for class in check_package(), warn if not present
  • Maybe provide more specific error message on what is invalid
• Add test for check_package()
• Make check_package() public, so other packages can use it