SALVIAS mappings: Map SiteCode/plot_code to locationevent.authorlocationcode because locationevent is now the place to store author-specific plot information

SALVIAS mappings: Fixed PlotID mapping to go to locationevent.sourceaccessioncode

VegBIEN: Renamed locationevent.authoreventcode to authorlocationcode to reflect that datasources usually use an author-defined code for a plot rather than a plot event

vegbien.sql: locationevent: Redid unique constraints to handle datasources that treat the authoreventcode as an authorlocationcode. Eventually, authoreventcode will be renamed to authorlocationcode.

vegbien.sql: locationevent: Redid unique constraints to work properly for all fully-specified combinations of keys

VegBIEN mappings: Mapped datasource name to new project.datasource. Fixes project duplicate elimination.

vegbien.sql: Renamed project.reference_id to datasource_id and pointed it to party, to match locationevent, etc.

VegBIEN mappings: Mapped current lat/long to centerlat/long as well so location duplicate elimination will work properly

xpath.py: Added support for common subpath after split path's {}

sql.py: put(): When encountering a DuplicateKeyException, use dict_subset_right_join to fill in explicit NULL values for columns which don't have data. This causes the database to use the UNIQUE constraint's index to look up the record, instead of relying on individual column indexes for the columns that did have data, which may or may not be available.