Datasets won't be re-loaded if their loading logic or SQL transforms have changed but their source data hasn't

[toolness]

As an example, even though I fixed #13, the derived tables for hpd_registrations still haven't been created because that dataset's data still hasn't changed, as it's only updated monthly.

Obviously this will be even worse for datasets that never change.

So it might be good if we provide some simple mechanism to force-reload a dataset.

[toolness]

Okay, it's not particularly well-documented right now, but I've created a foundation for fixing this in faa39ea.

Running python dbtool.py lastmod:reset <dataset> can now be used to reset the last-modified metadata of one or more datasets, so that the next time their loader runs, it will load the whole thing.