Enable optical stepping loop

[sethrj]

This activates (and fixes) the optical stepping loop by updating the track status at the beginning of the step. We don't need an "initialize vacancies" action like with the main stepping loop because we won't manage secondaries the same way (see #1538 (comment) ). Instead, the "initialize track" action will reset the status to "initializing" for any new tracks, and the "pre-step" action will set the status to "inactive" for any tracks that died in the previous iteration but weren't replaced with newly generated photons.

With this change we're actually stepping photons outside of the geometry! (With no physics or interactions.)

[github-actions]

Test summary

 3 901 files   6 012 suites   4m 15s ⏱️
 1 618 tests  1 589 ✅ 29 💤 0 ❌
20 006 runs  19 931 ✅ 75 💤 0 ❌

Results for commit 4292320.