Add option to checkpoint only when improvement is detected

[lesshaste]

Currently, checkpoints are saved based on checkpoint_interval (e.g., every 10 iterations), regardless of whether any improvement was made. This can be wasteful when the optimization is making slow progress.

Feature Request:

Add a configuration option checkpoint_on_improvement: bool that when set to True, only saves a checkpoint when a new best solution is found, rather than at fixed intervals.

Possible Implementation:

1. Config: Add checkpoint_on_improvement: bool = False to Config class in openevolve/config.py
2. Logic: Modify checkpoint trigger in openevolve/process_parallel.py (around line 667):

• If checkpoint_on_improvement is True, checkpoint only when database.best_program_id == child_program.id (improvement detected)
• If False, use existing interval-based logic

Example use case:

• Only save checkpoint when a new best is found
  checkpoint_on_improvement: true
• checkpoint_interval is ignored when above is true

This is especially useful when:

• Users only care about capturing improvements, not every N iterations
• Long-running jobs where disk space is a concern