xml_func.py: process(): Merge mergeable siblings recursively so that newly-mergeable children are also merged

xml_dom.py: Removed no longer used merge_same_name()

xml_func.py: process(): Recombining pieces of nodes that were split apart in the mappings: Only combine nodes that are explicitly marked as mergeable, to avoid unwanted merges. Refactored to use new xml_dom.merge_by_name().

mappings/VegX-VegBIEN.stems.csv: Marked aggregateoccurrence as mergeable, in preparation for switching to explicit merging only to avoid unwanted merges

xml_dom.py: Moved merge_adjacent() recursion and name checking into merge() so that other callers of merge() can take advantage of it, too. Added merge_by_name() and use it in merge_same_name(). Removed now-unused merge_adjacent().

xpath.py: get(): Support checking all children when the elem name is '*'

xml_dom.py: replace(): Assert that old node not removed from parent tree

xpath.py: Added is_all()

xml_func.py: process(): List out the xml_dom.NodeElemIter iterator so that it won't be affected by concurrent changes to the DOM tree

xml_func.py: process(): Merging children: Support merging non-adjacent children by using new xml_dom.merge_same_name(). This should eliminate the need to sort children of the same name next to each other in the mappings so that they will be merged.