sql_gen.py: as_Table(): Added schema param to use as default schema

inputs/SALVIAS: Switched to using CSV exports of the DB, so that staging tables could be created for column-based import

sql.py: run_query_into(): Added add_indexes_ param which causes the function to add indexes on the created table

sql.py: create_table(): Use new add_indexes()

sql.py: Added add_indexes()

sql.py: get_cur_query(): Fixed bug where strings.ustr() needed to be used instead of str() when ensuring that get_cur_query() returns a string

sql.py: cast(): Removed conditional checks for save_errors, since it's now always true if the function got passed the `not save_errors` special case

sql.py: cast(): Only convert errors to warnings if errors will be saved in errors_table, so that import will always be aborted if user supplied invalid values in the mappings, even if these values are passed through a relational function

sql.py: put_table(): Support inserting tables with all default values, by providing the pkey's default value for all rows so that the SELECT query has at least one column

sql_gen.py: is_table_col(): Check that input is a Col object