catmaid and trakem2 average over z

src/main/java/bdv/BigDataViewer.java

src/main/java/bdv/img/catmaid/CatmaidImageLoader.java

...n/java/bdv/img/catmaid/CatmaidVolatileIntArrayLoader.java

...ava/bdv/img/openconnectome/OpenConnectomeImageLoader.java

src/main/java/bdv/img/trakem2/TrakEM2ImageLoader.java

...n/java/bdv/img/trakem2/TrakEM2VolatileIntArrayLoader.java