Bugs + additional features in checkpoints

[haykh]

• walltime-based checkpoint dumping
• [REQUEST] Inconsistent Output Write Timing After Restarts #111 (will postpone this for now, since it's not really obvious what the ideal behavior must be)
• [REQUEST] Incorporating the name of the simulation into checkpoint/ name #94
• Output does not include some of the meta attributes after the restart

Additional fixes

• compatibility with gcc15 warnings/errors
• consistent usage of path_t = std::filesystem::path instead of std::string throughout the code
• memory footprint of particles & fields in .info fixed

[haykh]

@jmahlmann when you have a moment, could you test the walltime-based dump? it's not yet documented properly, but there is an example in the input file. Basically, you specify an additional parameter:

[checkpoint]
  ...
  # Write a checkpoint once after a fixed walltime 
  #   @type: string
  #   @default: "" (disabled)
  #   @note: The format is "HH:MM:SS"
  #   @note: Useful for long-running simulations to ensure that ...
  #   @note: ... the simulation can be resumed after allocation expires
  #   @note: Empty string means disables this (still writes checkpoints on interval)
  #   @note: This does not exit the simulation
  walltime = ""

e.g.

[checkpoint]
  walltime = "23:30:00"

[haykh]

@SChernoglazov I think I fixed the issue with outputs after the restart. this is ready to be tested.