schemas/vegbien.sql: locationevent: Made location_id required because every locationevent should have a location, even one with no locationdeterminations. This also avoids the creation of a parent locationevent when subplots are not being used.

mappings/VegX-VegBIEN.stems.csv: Removed _collapse where it's no longer needed because sql_io.put() handles that now. Note that each locationevent will get an empty commclass, whether or not there are any commdeterminations. This can later be used to add new commdeterminations.

schemas/vegbien.sql: commclass: Changed commclass_unique to COALESCE classnotes so that there is only one commclass for a locationevent when the commclasses are not separately named. (Currently classnotes is used as the class name field, commname being the name of the community itself.)

schemas/vegbien.sql: commdetermination: Made commconcept_id NOT NULL because it doesn't make sense to have a commdetermination on nothing. Note that the commname field in commdetermination is not used for making determinations (and may need to be removed to avoid confusion); commname.commname is used instead.

schemas/vegbien.sql: locationevent: Added COALESCE index on location_id for use by column-based import

mappings/VegX-VegBIEN.stems.csv: Removed _collapse where it's no longer needed because sql_io.put() handles that now. Note that each plantobservation will get an empty stemobservation, whether or not there are any stemtags. This can later be used to add further stemtags.

mappings/VegX-VegBIEN.stems.csv: Removed _collapse where it's no longer needed because sql_io.put() handles that now

schemas/vegbien.sql: location: Made datasource_id, sourceaccessioncode NOT NULL to ensure that all locations are uniquely identifiable by their datasource's unique key (sourceaccessioncode)

sql_io.py: put(): Handle NullValueExceptions by returning a NULL pkey, just like put_table() (column-based import) does

VegBIEN: Fixing import issue related to duplicate entries in tables with children, where when a new table entry duplicates an existing entry, the 1:1 tables of that table and those tables' children are not merged, causing them to become orphaned. It is described in detail at <https://projects.nceas.ucsb.edu/nceas/projects/bien/wiki/Import_issues#Merging-duplicates-with-children&gt;, including the rationale for this solution. Note that this is not a bug in column-based import, it applies to row-based import as well. This commit fixes the issue for locationevent->location in plots data, by also mapping locationevent's unique keys to location.sourceaccessioncode and setting location.datasource_id.