[HOPSWORKS-2009] Append Features to Feature Group schema

[moritzmeister]

I am not 100% in favour of adding a new method for every meta data update, however, as long as users can't update much, this should be ok.

Example usage:

houses_sold = fs.get_feature_group("houses_sold")
houses_sold.append_features(Feature("new2", type="abc", default_value="10.0"))
houses_sold.update_description("abc new")