add a notebook using the bagging model

this demo does the same thing as the minimal_dashboard, but with a different model. this model bagging consists in training 30 neuroquery models with randomly subsampled vocabularies and averaging their coefficients. it is a standard trick to gain a bit of robustness. however with this model we cannot decompose the prediction into smoothing + encoding anymore, and it is less obvious how to get good estimates for the coefficients' variance so this model is not meant to replace the one currently on neuroquery.org. neurodash therefore seems the right place to create an online demo