Variable definition is not robust enough in case one column in Master dataset is missing #111
Description
If MasterDataset is missing the column mapped to a variable, the code will break as each variable accesses the corresponding data series (i.e. pd.Series) directly from master_dataset (which is a DataFrame).
Alternatively, a get method should be used, so that if the series is not there, the Variable won't be included in the generated data.json file.
(See backend.datasets.generate_json.py)