Track ETC changes in a table

[hmainzer]

The ETC of an workpackage should be calculatable from the database at any time. To achive that all changes to the etc of an wp, besides efforts, should be tracked in a table.
One option would be to create a new table for this, or the work_effort table could be used, by adding a new boolean column, to differntiate between etc changes and efforts.
Finally the etc in the workpackage table should never be changed by anything besides a calculation from work efforts and etc changes.