split afterEachPlane() out of LoopbackHeuristic

src/main/java/bdv/ij/ExportImagePlusPlugIn.java

src/main/java/bdv/ij/ExportSpimFusionPlugIn.java

src/main/java/bdv/ij/ExportSpimSequencePlugIn.java

src/main/java/bdv/ij/export/Scripting.java

...ain/java/bdv/ij/export/tiles/CellVoyagerDataExporter.java