Fix save_binary TypeError on objects holding a Cluster

[krystophny]

Fixes #114.

sscha.Utilities.save_binary raised TypeError: cannot pickle '_thread.lock' object when saving a sscha.Relax.SSCHA object after a relaxation. The lock comes from Cluster.compute_ensemble_batch, which sets self.lock = threading.Lock() and never clears it, so any object referencing the cluster after a calculation cannot be pickled.

Instead of replacing pickle with dill, this adds __getstate__/__setstate__ to sscha.Cluster.Cluster. The lock is transient runtime state: it is dropped on save and reset to None on load (the same value __init__ assigns), and compute_ensemble_batch recreates it on the next run. Plain pickle now round-trips, existing binaries stay loadable, and the restored object remains usable (see also #105).

A regression test in tests/test_save_binary/ builds a minimal relax object from Examples/ensemble_data_test with a cluster in the post-calculation state, saves it with save_binary, reloads it with load_binary, and checks the restored state.

Verification

Before the fix:

$ python tests/test_save_binary/test_save_binary.py
Traceback (most recent call last):
  File "tests/test_save_binary/test_save_binary.py", line 50, in test_save_binary_relax_with_cluster
    sscha.Utilities.save_binary(relax, filename)
  File "Modules/Utilities.py", line 663, in save_binary
    pickle.dump(object, open(filename, "wb"))
TypeError: cannot pickle '_thread.lock' object

$ pytest -q tests/test_save_binary
FAILED test_save_binary.py::test_save_binary_relax_with_cluster - TypeError: ...
1 failed, 2 warnings in 0.95s

After the fix:

$ python tests/test_save_binary/test_save_binary.py
save_binary/load_binary round trip succeeded

$ pytest -q tests/test_save_binary
1 passed, 2 warnings in 0.91s

$ cd tests && OMP_NUM_THREADS=1 pytest -q -m "not release" --ignore=aiida_ensemble
23 passed, 1 skipped, 1 deselected, 29 warnings in 95.75s (0:01:35)

[mesonepigreco]

The edits are extremely local; the test suite also passes in the GitHub workflow.
It is safe to approve.

Thanks a lot! :)