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.

xml_dom.py: Added merge_same_name()

bin/map: ex_tracker: Don't add row_ct to iters count in column-based import (by_col) because errors are not done by row, so a % of rows affected is not meaningful

sql_io.py: put_table(): NullValueException: Missing mapping for NOT NULL column: Raise error if there is no default input column to fill in the rows with. Missing mappings for required columns are a normal circumstance with path mappings (plantname, namedplace) because the mappings include intervening levels that may not be specified by the datasource. These mappings have default input columns for the intervening levels, so for them, this will continue to not be flagged as an error.