hy_test_db: Get the location of the HYDAT database
Description
The full HYDAT database needs to be downloaded from download_hydat, but for testing
purposes, a small test database is included in this package. Use
hydat_path = hy_test_db() in hy_* functions to explicitly use the test database;
use hydat_path = hy_downloaded_db() to explicitly use the full, most recent
downloaded database (this is also the path returned by hy_default_db()).