Make sure time boundary doesn't prevent secondary particle creation

[paulromano]

Description

This PR fixes a bug related to when a time boundary is in use that can result in secondary particles not be simulated. The sequence of events that leads to this is as follows:

• In Particle::event_advance, a time boundary results in the particle's weight getting set to zero.
• However, this doesn't stop the particle from undergoing a collision because there is no conditional check that the particle is still alive when Particle::event_collide is called. This can result in zero-weight secondaries getting created.
• Immediately after that, Particle::event_revive_from_secondary sets the particle's state to one of the just-created zero-weight secondaries, which then causes the event loop to terminate because Particle::alive() is false.
• Thus, any other secondary particles that were still present in the secondary bank don't get simulated.

The solution is to just add a check after calling event_advance() that the particle is still alive before processing a surface crossing or collision.

Fixes #3039

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[cfichtlscherer]

Looks all good for me - amazing that you figured out what was going on. I've been looking at it for hours....