Additional tools for working with IBE, including SIA access

PR includes tools for listing the valid / available data sets, missions, tables and a tool for querying the SIA component of IBE (which is the only programmatic way to get image access, as far as I can tell)
Not directly, but we could use `conf.MISSION`... or `mission=None` and `if mission is None: mission=self.MISSION`.  Preference?