Multi-point prompt for auto-sam, create scalar label source, fixes

[cmhulbert]

features:

• If multiple connected components are found during auto-SAM in ShapeInterpolation, generate a prompt point per component
• create scalar label sources
• replace auto sam slices completely (instead of adding to them) when manually entering SAM at an auto-sam-only slice

fix:

• refresh interpolation when exiting manual SAM tool
• don't refresh interpolation untill all bisect slices are done during bisect all
• invalidate SAM request key if cancelled
• ignore null property value during serialization/deserialization