Add/enhance utilities in Query class

[JoanneBogart]

We have a function Query.get_all_columns(self) which does indeed return the names of all columns from all tables. But that can be overwhelming. We could add an optional argument table, defaulting to None to give current behavior.

Then to make that more useful we could add
Query.get_all_tables(self)
which would return a list of table names for the current schema.