This method will query blob data that is a serialized array of of {@link DataTable\Row}'s and
unserialize it.
If multiple sites were requested in {@link build()} or {@link factory()} the result will
be a {@link DataTable\Map} that is indexed by site ID.
If multiple periods were requested in {@link build()} or {@link factory()} the result will
be a DataTable\Map that is indexed by period.
The site ID index is always first, so if multiple sites & periods were requested, the result
will be a {@link DataTable\Map} indexed by site ID which contains {@link DataTable\Map} instances that are
indexed by period.