HistoryDag subclass defaults

[willdumm]

This PR proposes a convenient interface for defining default arguments for weight counting-related methods of HistoryDag subclasses.

• Default arguments are read from a subclass's _default_args dictionary, and applied by the method decorator dag.get_default_args
• Using get_default_args on a method causes positional arguments to be positional-only, and keyword arguments to be keyword-only
• get_default_args takes a list of arguments to be potentially overridden, as well as a count of (non-self) positional arguments.
• the convenience function dag.convert is added as a wrapper for HistoryDag.from_history_dag.

[willdumm]

replaces #62 to avoid rebase