Add callbacks to the optimisation loop

[therooler]

Users currently have no way to hook into the running optimisation loop — e.g. to implement early stopping based on a custom criterion, log to an external system, or visualise intermediate pulses.

The History class (history.py) covers post-hoc logging but requires the full run to finish.

Proposed change:
Add an optional callbacks argument to Geope.optimize — a list of callables with the signature:

def cb(step: int, history: History, geope: Geope) -> bool:
    ...  # return False to request early stop

Callbacks are called after each optimisation step (after add_parameters in the main loop, geope.py:615–647). If any callback returns False the loop exits cleanly.

Acceptance criteria:

• callbacks=[cb] is accepted and called each step.
• Returning True from a callback terminates the loop early.
• Default (callbacks=None) preserves current behaviour with zero overhead.
• At least one test covering early-stop and logging callbacks.