fix restart trajectory for merge_traj (#1111)

When turning on the merge_traj function, if the procedure is interrupted
in the run_model_devi stage, the old trajectory file will be overwritten
after it is rerun. The incomplete trajectory file will cause the
following error in the next post_model_devi stage:
`.../site-packages/dpdata/lammps/dump.py:176: VisibleDeprecationWarning:
Creating an ndarray from ragged nested sequences (which is a
list-or-tuple of lists-or-tuples-or ndarrays with different lengths or
shapes) is deprecated. If you meant to do this, you must specify
'dtype=object' when creating the ndarray.
system['coords'] = np.array(system['coords'])
DeepModeling
`
This PR repaired this question.

---------

Signed-off-by: kiwi <46419582+wankiwi@users.noreply.github.com>
Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>
Co-authored-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

dpgen/generator/lib/lammps.py

@@ -108,6 +108,7 @@ def make_lammps_input(
     model_devi_merge_traj = jdata.get("model_devi_merge_traj", False)
     if model_devi_merge_traj is True:
         ret += "dump            1 all custom ${DUMP_FREQ} all.lammpstrj id type x y z fx fy fz\n"
+        ret += 'if "${restart} > 0" then "dump_modify     1 append yes"\n'
     else:
         ret += "dump            1 all custom ${DUMP_FREQ} traj/*.lammpstrj id type x y z fx fy fz\n"
     ret += "restart         10000 dpgen.restart\n"