Add 'store_snapshot' method into aggregators
Some data won't fit neatly into timeseries databases (e.g. InfluxDB) such as ROC curves or PSDs which are inherently 2-dimensional. However, we usually only care about snapshots of these quantities, so we could store a JSON representation to somewhere in /public_html so that serve.py can read in JSON directly from a url and format it for use in plots.