db_xml.py: put(): Raise an error if there are multiple fields with the same name, instead of silently overwriting the first with the second. This generally indicates the need to use `:[@merge=1]` on the fields in question.

dicts.py: Added OnceOnlyDict and helper exception KeyExistsError

dicts.py: DictProxy: Added default value for inner param to facilitate creating empty wrapped dicts

bin/map: out_is_db: row-based mode: Debug-log the processed XML tree produced by xml_func.process()

sql_io.py: put_table(): Fixed bug where Missing mapping for NOT NULL column errors should actually be warnings because sometimes the mappings include extra tables which aren't used by the dataset

sql_io.py: put_table(): Fixed bug where Missing mapping for NOT NULL column errors should actually be warnings because sometimes the mappings include extra tables which aren't used by the dataset

schemas/vegbien.sql: aggregateoccurrence: Added UNIQUE INDEX that makes an aggregateoccurrence unique within a taxonoccurrence. When the sourceaccessioncode isn't specified (as for individual organisms data, where this goes in plantobservation and taxonoccurrence), this ensures a 1:1 relationship between aggregateoccurrence and taxonoccurrence.

schemas/vegbien.sql: taxonoccurrence: Added UNIQUE INDEX that makes a taxonoccurrence unique within a locationevent. When the sourceaccessioncode isn't specified (as for specimens data), this ensures a 1:1 relationship between taxonoccurrence and locationevent.

mappings/VegX-VegBIEN.stems.csv: binomial (full) plantname: Also mapped to an alternative for taxonoccurrence.sourceaccessioncode, for aggregate plots data that distinguishes taxonoccurrences only by plantname (such as CVS)

exc.py: e_msg(): Fixed bug where exceptions with nothing in e.args (such as StopIteration) caused a failed assertion. Fixed bug where exceptions with multiple values in e.args (such as certain IOErrors) caused a failed assertion.