Creating temp dbs

[wendy-aw]

Made changes to eval.py to accommodate creation of temporary postgres databases not found in the standard 7 dbs of defog-data.

There is now an option to create (and drop) temporary DBs if the db_name contains the suffix "_temp". The table_metadata_string col containing the DDL statements for this temp DB must also be provided in the same data/your_custom_qns.csv file. Note that it's important that the DDL statements contain the insertion of VALUES so that we can evaluate on non-empty dataframes.

Additional note: to prevent race conditions during creation and dropping your temporary databases, it's best to give each question a unique db_name like temp_db_name = db_name + question + "_temp" or temp_db_name = db_name + row_index + "_temp"

[wongjingping]

passed on my end, thanks!