opened 09:05AM - 07 Dec 17 UTC
When aggregating images with the *GroupBy* node, it would be useful to also have… a "Sum Image" option in addition to Min/Mean/Max Image.
How about adding `org.knime.knip.base.data.aggregation.ImgSumOperator` modeled after [the other aggregation operators](https://github.com/knime-ip/knip/tree/master/org.knime.knip.base/src/org/knime/knip/base/data/aggregation)? (As for the other aggregation methods, the caller should be responsible for using the correct desired type.)
