First idea concerning GroupBy. Likely to change.
Implemented a model for the group by to help me think.
Merge branch 'master' of https://github.com/koradir/dmdb2014
Model finally working as intended.
Now starting to implement it.
Merge branch 'master' of https://github.com/koradir/dmdb2014
Added a "changeSource"-method to all Operators. Future Operators must also implement this.