Add HistoryDagFilter Object

[willdumm]

This PR contains lots of improvements for convenient modification of HistoryDag objects

• adds a HistoryDagFilter object, extending AddFuncDict to include a function describing optimality, and implements a filtering syntax using &.
• adds the method HistoryDag.weight_range_annotate for automatically computing the minimum and maximum history weight in a DAG
• adds the method HistoryDag.filter_nodes and HistoryDag.filter_node for finding nodes that make a passed function True
• provides a more convenient interface for creating weighted hamming distance edge functions in historydag.parsimony
• adds an option to not trim the DAG after calling parsimony.sankoff_downward.

[willdumm]

The most recent commits change the proposed & syntax for filtering to instead use indexing syntax, in case we want & to mean intersection (of tree sets) between history DAGs in the future.