Activity data changes: Logfile

[tmillross]

An option for tracking data changes, as related to #10.

1. Globally maintained list of changes per session
2. Just an append-only log: no further functionality required
3. For LCA practitioner – this could provides a complete overview of their original process development activities (if conducted through AB)
4. Could be viewed in AB for supplementing with sources or notes etc. But probably not necessary as they could use an text editor if this is desired
5. Could be stored on disk as json file, perhaps in same directory as ab_settings.json
6. User should be informed that the log exists, and how it can be helpful to them
7. Data to be appended on each modification:
   i. Timestamp
   ii. BW project
   iii. db
   iv. activity guid
   v. exchange id (optional)
   vi. data key
   vii. original value (None if insert)
   viii. new value
   ix. data change type (maybe unnecessary)