Add unit tests for `convert_sql_to_csv_summary`

# Add unit tests for `convert_sql_to_csv_summary`

Add unit tests (in `h2integrate/postprocess/test`) for the post-processing function `convert_sql_to_csv_summary()` in `h2integrate/postprocess/sql_to_csv.py`. Also add documentation on this method or its usage to `docs/user_guide/postprocessing_results.md`.