ensure_db_available

MCEq.config.ensure_db_available()

Download the MCEq database if not already present.

Called by MCEqRun.__init__ so that the download is deferred until the database is actually needed. This allows tests (and other callers) to override config.mceq_db_fname before a download is attempted.

The integrity check only applies to the default database; non-default files are accepted as-is if they exist.