Finds the chain containing the given edge, and returns the position of that edge as a (chain_id, offset) pair.
REQUIRES: shape.chain(pos.chain_id).start + pos.offset == edge_id REQUIRES: shape.chain(pos.chain_id + 1).start > edge_id
where pos == shape.chain_position(edge_id).
See Implementation
Finds the chain containing the given edge, and returns the position of that edge as a (chain_id, offset) pair.
REQUIRES: shape.chain(pos.chain_id).start + pos.offset == edge_id REQUIRES: shape.chain(pos.chain_id + 1).start > edge_id
where pos == shape.chain_position(edge_id).