Rename subsplit DAG elements

[matsen]

Here are some proposals. Add yours as well!

• dag root -> universal ancestor (ua)
• rotated -> left, and sorted -> right
• PCSP -> edge
• fake subsplit -> leaf subsplit

Let's discuss and come to consensus before making any change.

[matsen]

This isn't quite a naming issue, but I wonder how we decided to have the DAG root have bitset 1111|0000 rather than having the zeroes first. Did we want to have things come down through the rotated edge? cc @shokiami @tanviganapathy

[davidrich27]

• Chunks (and child in subsplit constructor) -> clades

• Make left and right clade as enumerated types for subsplits?

• Make sister, focal and left-child as enumerated types for edges (formerly PCSP)?

• Not a rename, but I would like to put forward the idea of Internal vs External nodes and edges. Internal nodes are subsplits which do not go out to an extremity (a leaf or a root). Internal edges are edges for which neither the parent nor child are external.