allow only specific columns in superset dataset

[sheralim012]

Closes #1570

Only allow specific columns in the apache superset dataset.

[cbunicef]

Thanks Sher. I think it's also good that we are specifically listing the columns, that will prevent any new columns being included automatically.

[Alihassanc5]

@istride According to this a modification has been made to the API for creating virtual datasets and has been integrated, but the feature have not yet been released. We will update this accordingly after the release.

[istride]

@sheralim012 Eventually, it might be possible (and more Pythonic) to combine all the code in 'questionnaires/superset' into a single superset.py file and that is the direction I prefer to go in. For now, please move ALLOWED_COLUMNS from 'constants.py' to 'questionnaires/superset/__init__.py' and update the imports accordingly.

Feel free to merge once done.

[istride]

...a modification has been made to the API for creating virtual datasets ... but the feature have not yet been released. We will update this accordingly after the release.

@Alihassanc5 sounds good. I'll leave it up to you to keep track of that and create an issue when it's time to update.