error for ncpu > 1 in descriptor

With ncpu > 1 in the descriptor I get:
=========================== version 0.2.2 =============================

Descriptor parameters:
type        : SO4
Rc          : 12.0
cutoff      : cosine
lmax        : 4
normalize_U : False

1 structures have been loaded.
Computing the descriptors...
Traceback (most recent call last):
  File "/home/train_potentials.py", line 91, in <module>
    ff.run(mode='train', TrainData=train_data)
  File "/opt/python/python-3.10.10/lib/python3.10/site-packages/pyxtal_ff/__init__.py", line 265, in run
    trainDB.store(TrainData, self._descriptors, True, self.path+'ase.db')
  File "/opt/python/python-3.10.10/lib/python3.10/site-packages/pyxtal_ff/utilities/__init__.py", line 111, in store
    self.add(function, data)
  File "/opt/python/python-3.10.10/lib/python3.10/site-packages/pyxtal_ff/utilities/__init__.py", line 147, in add
    for i, d in enumerate(p.imap_unordered(func, _data)):
  File "/opt/python/python-3.10.10/lib/python3.10/multiprocessing/pool.py", line 873, in next
    raise value
  File "/opt/python/python-3.10.10/lib/python3.10/multiprocessing/pool.py", line 540, in _handle_tasks
    put(task)
  File "/opt/python/python-3.10.10/lib/python3.10/multiprocessing/connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "/opt/python/python-3.10.10/lib/python3.10/multiprocessing/reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
TypeError: cannot pickle '_gdbm.gdbm' object
MaterSim / PyXtal_FF

error for ncpu > 1 in descriptor #58
