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.

sql.py: flatten(): Documented that shouldn't cache query because the temp table will usually be truncated after use

sql_gen.py: merge_not_null(): For clarity, use to_text() to represent NULL as the string 'NULL' instead of as the null sentinel for the column's type

sql_gen.py: Added to_text() and helper value null_as_str

mappings/VegX-VegBIEN.stems.csv: plantobservation: sourceaccessioncode, authorplantcode: Removed no longer needed mapping to specimenreplicate.sourceaccessioncode, since specimenreplicate for plots data is now identified by its plantobservation fkey, without needing its own sourceaccessioncode

sql_io.py: put_table(): ignore_cond(): Fixed bug where if is_literals, need to return NULL, instead of trying to filter invalid rows out of a nonexistant input table

mappings/VegX-VegBIEN.stems.csv: Replaced "/}" (with unnecessary "/") with "}"