Fix simplify

[HuangJiameng]

When there is no candidate, ZeroDivisionError: float division by zero is raised. Here the bug is fixed.
However, it seems that the current devel branch can't make it through unittest. But it has nothing to do with this PR.

FileExistsError: [Errno 17] File exists: '/root/dpgen/tests/data/abacus.in/Cu_ONCV_PBE-1.0.upf' -> 'cuw.fcc.01x01x01/00.place_ele/sys-0003-0001/Cu_ONCV_PBE-1.0.upf'

[codecov-commenter]

Codecov Report

Merging #767 (89a446b) into devel (dc57c9a) will increase coverage by 0.86%.
The diff coverage is 36.88%.

@@            Coverage Diff             @@
##            devel     #767      +/-   ##
==========================================
+ Coverage   33.94%   34.80%   +0.86%     
==========================================
  Files          86       96      +10     
  Lines       15424    16640    +1216     
==========================================
+ Hits         5235     5791     +556     
- Misses      10189    10849     +660     

Impacted Files	Coverage Δ
dpgen/auto_test/Lammps.py	30.94% <0.00%> (ø)
dpgen/data/reaction.py	0.00% <0.00%> (ø)
dpgen/data/surf.py	67.33% <0.00%> (+0.13%)	⬆️
dpgen/data/tools/cessp2force_lin.py	0.00% <0.00%> (ø)
dpgen/data/tools/create_random_disturb.py	0.00% <0.00%> (ø)
dpgen/dispatcher/ALI.py	0.00% <0.00%> (ø)
dpgen/dispatcher/DispatcherList.py	0.00% <0.00%> (ø)
dpgen/dispatcher/LSF.py	9.35% <0.00%> (ø)
dpgen/dispatcher/SSHContext.py	22.76% <0.00%> (ø)
dpgen/dispatcher/Shell.py	14.45% <0.00%> (ø)
... and 38 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dc57c9a...89a446b. Read the comment docs.