sc_start/sc_end - not just edges

[mdsumner]

these need to come out for all models, and not be documented with sc_edge/sc_node:

• [x] PATH return poly/line start end of all paths
• [ ] PATH0 (needs to be implemented without using PATH)
• [x] SC, SC0 - current behaviour
• [ ] TRI, TRI0 - as per PATH for polygon
• [ ] ARC as per PATH for lines
• [ ] document as general helper funs

also see https://github.com/hypertidy/silicate/issues/88#issuecomment-465845365