Snarl child count

[xchang1]

Changelog Entry

To be copied to the draft changelog by merger:

Description

Add a unit test for new distance index function counting the number of children in a snarl

[adamnovak]

It looks like the Gitlab tests failed here because vg.pb.h didn't generate right; the CI run says it is shorter than expected and then doesn't have Alignment defined. See: https://ucsc-ci.com/vgteam/vg/-/jobs/98298#L2162

I'm not sure why that would happen, unless that run picked up a bad version if the Protobuf compiler or unless there's something badly wrong with the isolation between builds. I'm rerunning it to see if it happens every time.