Add edit reset-midpoints command that will remove midpoints from all or selected edges.

Proposed usage:

poietic edit reset-midpoints [EDGE...]

Implementation Notes

• resolve ID references to actual ID objects
• throw error if any of the IDs does not exist
• change only those that have midpoints set

let trans: TransientFrame

for edge in edges {
    guard edge["midpoints"] != nil else {
        continue
    }
    var mutable = trans.mutate(edge.id)
    mutable["midpoints"] = nil
}
...

• Commit only if there are changes