Updated imagen submodule reference
Removed dependency on topo.misc.attrdict in topo/base/sheet.py
Moved cyclic_range for maps from the NdMapping to the SheetView
Removed unused import of FeatureCurveCommand
Fixed missing variable declarations and cleaned up code
Fixed issue with gradientplots accessed from the GUI
Cyclic_range is now accessed through the SheetView
Cleaned up vision.py, not clear whether current indentation levels are correct

This file should be considered experimental and should ungergo some review.