Move repeated consumer logic to base IGWNAlertConsumer

Add plot_snr_series and plot_psd_series functions.
Default alert_callback will save all alerts and updates to a directory.
Add misc consumer for saving/uploading psd_series and snr_series.
Add save-to-file option for other analysis functions.

callback_alert is a bad name and should be changed because it's not passed to a function so it's not strictly a callback function (I think).
I've also moved the parse payload / debug event_id / save payload logic to the base process_alert function. I think all consumers will want to use it.