Change to PanSN "scaffolds" with colon and dash coordinate ranges

[adamnovak]

This changes the notation for subpaths, and adds some queries related to "scaffolds" that paths are on.

It also eliminates phase blocks. We are going to use possibly-fake coordinate ranges instead.

Problems:

• Without phase blocks in the name you can't tell if a name is a haplotype name or a reference name by looking at it. We might need to add the list-of-reference-samples mechanic to PackedGraph and friends, or some other way to store the bit.
• I have non-path-metadata-related code for parsing and dealing with a region type. But the region is just another way of parsing out the subpath from the rest of the name. Maybe some of that can/should belong to PathMetadata?
• I'm not sure on the region accessors. I have one to get the region for a path in PathMetadata, but that means PathHandleGraph needs a path length query. Maybe the calls for working with regions need to be part of PathPositionHandleGraph, and I should have only scaffold-based calls on PathMetadata?
• I don't actually have the get-path-step-at-these-scaffold-coordinates logic that we might need, or the see-if-step-is-in-region-set logic we would need for surject or rGFA.

[adamnovak]

I abandoned this and right now we're using some utility functions in vg itself to deal with going between base path information (where a base path has a string name that identifies it and may or may not exist as a path in a graph with a handle) and subpaths (identified by handles).

https://github.com/vgteam/vg/blob/62031cba5f83353efcb52c7a2fe2684e8a887743/src/path.hpp#L397-L452