Add a private method to validate the metadata for single and multi-table reports.
For single table the metadata must be a dict with a 'column' key.
For multi-table the metadata must be a dict with a 'table' key. All table must also have a 'column' key
Add a private method to validate the metadata for single and multi-table reports. For single table the metadata must be a dict with a 'column' key. For multi-table the metadata must be a dict with a 'table' key. All table must also have a 'column' key