This function provides a default database connection for
`rfishbase`. Note that you can use `rfishbase` with any DBI-compatible database
connection by passing the connection object directly to `rfishbase`
functions using the `db` argument. `default_db()` exists only to provide
reasonable automatic defaults based on what is available on your system.
`duckdb` or `MonetDBLite` will give the best performance, and regular users
`rfishbase` will work with the built-in `RSQlite`, and with other database connections
such as Postgres or MariaDB, but queries (filtering joins) will be much slower
on these non-columnar databases.
For performance reasons, this function will also cache and restore the
existing database connection, making repeated calls to `default_db()` much
faster and more failsafe than repeated calls to [DBI::dbConnect]