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

sql.py mk_select(), sql_gen.py Join.to_str(): Fixed bug where conditions needed to be wrapped in () before being AND-ed together to ensure the proper operator precedence

sql.py: put_table(): Add index on columns with invalid values to enable fast filtering

sql.py: Added index_col()

sql.py: put_table(): Add pkey on returned pkeys table to enable fast joins

sql.py: Added index_pkey()