[BUG] system cannot be appended to multi-systems if it has been dumped in mixed type format

[wanghan-iapcm]

Bug summary

system cannot be apppended to multi-systems if it is dumped to a mixed type format

dpdata Version

0.2.19

Input Files, Running Commands, Error Log, etc.

does not apply

Steps to Reproduce

import dpdata
import numpy as np

system_1 = dpdata.System()
system_1.data["atom_names"] = ["foo", "bar"]
system_1.data["atom_types"] = np.array([1, 0], dtype=int)
system_1.data["atom_numbs"] = [1, 1]
system_1.data["cells"] = np.array(
    [10, 0, 0, 0, 10, 0, 0, 0, 10], dtype=float
).reshape(1, 3, 3)
system_1.data["coords"] = np.array(
    [0, 0, 0, 0, 0, 1], dtype=float
).reshape(1, 2, 3)

system_1.to('deepmd/npy/mixed', 'tmptmp')

ms = dpdata.MultiSystems()
ms.append(system_1)

the error raised is

Traceback (most recent call last):
  File "/root/Manual/Washed-restart/replicateerr/wanghan.1/test.py", line 18, in <module>
    ms.append(system_1)
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/system.py", line 1485, in append
    self.__append(system)
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/system.py", line 1497, in __append
    system = system.copy()
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/system.py", line 434, in copy
    return self.__class__.from_dict({"data": deepcopy(self.data)})
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/system.py", line 397, in from_dict
    return cls(**decoded)
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/system.py", line 196, in __init__
    self.check_data()
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/system.py", line 224, in check_data
    dd.check(self)
  File "/opt/deepmd-kit-3.0.0/lib/python3.10/site-packages/dpdata/data_type.py", line 120, in check
    raise DataError(
dpdata.data_type.DataError: Length of real_atom_names is 2, but expected 1

Further Information, Files, and Links

No response