sql.py: put_table(): Factored out mk_select() calls in calls to run_query_into_pkeys() into new helper function insert_into_pkeys()

sql.py: put_table(): run_query_into_pkeys() calls use order_by=None in their select statements because there is a pkey, so order (row #) does not matter

db_xml.py: put_table(): Subset in_table if limit != None or start != 0. start param defaults to 0 again to avoid subsetting the table when starting from row 0 (with no limit).

db_xml.py: put_table(): Don't pass limit, start recursively, because the table subsetting will happen only once in the first invocation of the function. Moved limit, start params to end since they are not passed recursively. start param no longer defaults to 0 because this is not needed since sql.put_table() now sets start to 0 where needed.

sql.py: put_table(): Removed limit and start params because they were never fully implemented, and because it's simpler to just have the caller subset their input table

lists.py: Added uniqify()

sql.py: Moved mk_flatten_mapping(), flatten() to Basic queries section since they don't involve database structure info

sql.py: put_table(): Use single quotes rather than double quotes around strings where possible

schemas/functions.sql, vegbien.sql: Changed CAST-related relational functions to return NULL on data exceptions and convert the exceptions to warnings. This helps column-based import by mapping invalid values to NULL instead of aborting the whole query on the first invalid value.

sql.py: index_col(): Cache the query so it doesn't try to add an index on the same column multiple times