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.

sql.py: DbConn.run_query(): Separate the data source comment from the query with a tab in the executed query but a \r in the logged query, so that the query will be shown on the same line as the data source comment in pg_stat_activity, but be hidden by the following line when cating the file and be put on a separate line when viewed in a text editor. This causes the first line of the query to be at the left edge when the log file is viewed, so that it looks more natural.

README.TXT: Data import: Import data into VegBIEN: Added command to use for column-based import

schemas/vegbien.sql: locationevent: Allow a locationevent to be uniquely specified by its location (which is now datasource-scoped) and start date

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 specimenreplicate->...->location, by also mapping specimenreplicate's unique keys to location.sourceaccessioncode and setting location.datasource_id.

schemas/vegbien.sql: locationevent: Allow to be uniquely specified by location_id. This is useful for specimens data where there is one location for every locationevent.

schemas/vegbien.sql: location: Added datasource_id and sourceaccessioncode so locations can be uniquely specified by the input datasource, rather than being created automatically for each locationevent

schemas/filter_ERD.csv: Add back taxondetermination->taxonoccurrence fkey because that has been replaced by a trigger in the SQL