Add new track status and support user "initialization"

[sethrj]

This defines an "error" status (to supporting a refactor of #1290) and an "initializing" status, which is set whenever a track slot starts its life. A new user_start action order allows users to set custom data that applies only to new tracks. I split the Stepper test into two, one of which requires Geant4 and the other which uses built-in data.

Changes:

• Rename post_post to user_post, and post_pre to user_pre
• "Not inactive" and "alive" are now different
• Slightly changed TrackSortUtils

Do we also want to add a Stepper output for "errored" tracks? 🤔

[amandalund]

I think this is a good idea! Before I look too closely, are the stepper tests a straight up refactoring, or did you make changes?

[sethrj]

The stepper tests basically got moved. There's one or two new tests in the Stepper, and almost everything got directly moved to StepperGeant which you can ignore

[amandalund]

Looks good to me, thanks @sethrj!