Add an '--unload' option/subcommand to nycdb

[nycdb]

Add an '--unload' option/subcommand to nycdb

[aepyornis]

Sure, why not! Yup, it'd mostly be just dropping tables and functions.

By the way, you don't have to parse the sql files, verify.py has a list of tables that are created for each dataset (including the derived ones). I'll admit, though, that it's not the most obvious place for that constant, so perhaps we should put it somewhere else or include that information in the yaml files instead.

[toolness]

Oooh yeah!! I really like the idea of putting verification-related metadata in the YAML file. Verification isn't documented in the ADDING_NEW_DATASETS.md guide and I have no idea how to do it... and to the extent that one of the best sources of ad-hoc documentation on our datasets so far is the YAML files, this would make it easier for folks to understand what derived tables are created for datasets!