Hide hierarchical SQLite example under main

I'm moving this under main so that it doesn't run with "test_examples".
This could be set up like the other examples, with a run definition that
allows for a short version, but I'd prefer not to for a couple of
reasons.

1. Everything aside from the SQLite backend is the same as
   "hierarchical.py", so it isn't testing much more for the time added
   to the run.
2. This results in an SQLite file, so a cleanup should be added
   somewhere if it is run with "test_examples".