Improve auto-adding of save_result

[soxofaan]

related to #623, #401, #583, #391

• Check whole process graph for pre-existing save_result nodes, not just final node
• Disallow ambiguity of combining explicit save_result and download/create_job with format

[soxofaan]

todo:

• [x] changelog
• [x] port to vectorcube class too