Track changes on the processing object.

[nicmostert]

It's becoming clear that it is important to track all data changes on the processing object. We can compare raw and processed data to see all changes, but a more granular description of all changes will be really good for the stats we can reconstruct out of that after the fact.

I can possibly see a reason for some sort of data structure that saves the following:

• [x] Type: Gap removal, ramp correction, spike removal, whatever.
• [ ] Range: Begin and end date? Surely this should be done in blocks.
• [ ] Colour for plotting purposes?
• [x] Some tags, maybe a category system? Destructive, transformation, substitution, etc?
• [ ] Note: Dependent on the change being made. For a substitution maybe the source file/site? Shift amounts, ramp equations, other notes, reasons for change, etc.

[nicmostert]

Initial support for this added in 0.6.0