Fix old method names using names from parsimony-plateau paper

[willdumm]

Lots of method names are poorly chosen, and do not match the terminology later chosen in the parsimony-plateau paper. Here are some examples

under_clade should be clade_union
partitions -> child_clades
is_root -> is_ua_node
node_self -> empty_copy
clade tree -> history
get_trees -> get_histories
count_trees -> count_histories
is_clade_tree -> is_history
_get_subtree_by_id -> _get_subhistory_by_id
_get_clade_tree -> _get_subhistory
utils._cladetree_method -> utils._history_method
postorder_cladetree_accum -> postorder_history_accum

Exposed methods which require renaming will be preserved, with the old name calling the method with the new name. These old method names preserved for compatibility should be ignored in the documentation, and should no longer be called by any internal code.