SyncedSliceSources now support partial syncing
adopted sync_along parameter in dirtypropagation test
support for through values caller override
Merge branch 'master' into multichannel

Conflicts:
	volumina/pixelpipeline/imagepump.py
NavCtrl now contains imagepumps instead of only syncedSliceSource. Prerparation for per layer channel selection.
added layerChangeChannel() to navigationControler; gui multichannel support still missing
made volumeeditor multichannel-ready