db_xml.py: _put_table_part(): Support no in_table, for iterations with only literal values

sql_io.py: put_table(): is_literals: When ignoring all rows, return default value instead of always None

db_xml.py: put_table(): Removed parent_ids_loc and next params since these are only used in the recursion

db_xml.py: put_table(): Split into an outer function that sets up the database environment and subsets in_table, and a (recursive) inner function that imports the data

db_xml.py: put_table(): Subsetting and partitioning in_table: Documented that it's OK to do this even if table already the right size because it takes <1 sec

sql_io.py: put_table(): Use is_function where caller-provided is_func was used, since is_function determines whether something is a function based on whether it actually exists as a SQL function instead of just whether its name starts with "_". Removed now-unneeded is_func param.

sql_io.py: put_table(): Added col_defaults param and use it if there's a missing mapping for a NOT NULL column. This requires callers passing arguments by position to add an empty value for this parameter.

bin/map: by_col: Only clear errors table if doing full re-import starting from row 0, not if restarting import at a later row

input.Makefile: Import to VegBIEN: Fixed bug where `&>>` was used to append stdout and stderr to the log file, but is not supported on Mac OS X. Replaced with `&>` (overwrite instead of append) because log file is unique by date/time the import runs, so there won't be an existing log file that would be overwritten.

schemas/vegbien.sql: Added datasource_id to all tables with a sourceaccessioncode (and corresponding *_unique_datasource constraint on these columns) so they can be directly looked up using just the input table's own fkey to parent. This will enable loading hierarchical (plots) data without "breadcrumbs", a huge benefit! Also added sourceaccessioncode wherever there was a datasource_id, to standardize on these names as being the columns that link directly to the input table rows.