Add debug output for track states

[sethrj]

While debugging an exception in the status checker I got tired of trying to dig into the raw state data to access the current track's debug information. This adds JSON I/O, and lldb-callable debug_print functions, for the track.

There's now a hacky piece of code to set the "currently executing params" from the action sequence. @pcanal you're the resident expert on global variables, so perhaps you could take a look at the global variable? I only intend this functionality to be used in single-thread mode for interactive debugging, so perhaps it doesn't need to be thread-local (and it might be bad if we do thread-local because the main thread can catch an exception run by an openmp thread?). Later we might extend the JSON I/O to allow KernelContextException to merge the core track state with the core params data without a global variable/lock...

Calling from an Executor that has an assertion failure:

(lldb) call celeritas::debug_print(track)
{
 "geo": {
  "dir": [
   0.9998302826766889,
   0.010529089939196719,
   0.015117675340624488
  ],
  "is_on_boundary": false,
  "is_outside": false,
  "pos": [
   -2.135075225174846,
   0.0,
   0.0
  ],
  "volume_id": "inner@0x60000350ada0"
 },
 "particle": {
  "energy": [
   96.45131706860465,
   "MeV"
  ],
  "particle_id": "electron"
 },
 "sim": {
  "along_step_action": "along-step-neutral",
  "event_id": 2,
  "num_steps": 0,
  "parent_id": 0,
  "status": "alive",
  "step_length": null,
  "time": 6.626225658693907e-10,
  "track_id": 1
 },
 "thread_id": 26,
 "track_slot_id": 26
}