example notebooks to source data from CONFIG.mail_path

[sbenthall]

We have a standard way of configuring a path for email archives: it's the mail_path value in the config file.

This is used by the mail collecting scripts.

It is not currently used by all the example notebooks. Instead, the example notebooks mostly use ad hoc, system specific paths based on whomever developed the notebook.

We should standardize on mail_path