add default cells when loading hdf5 data

[njzjz]

dpdata requires cells even in nopbc data.

[codecov-commenter]

Codecov Report

Merging #262 (2b1f81b) into devel (d9e400f) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##            devel     #262      +/-   ##
==========================================
- Coverage   82.09%   82.06%   -0.03%     
==========================================
  Files          60       60              
  Lines        5150     5153       +3     
==========================================
+ Hits         4228     4229       +1     
- Misses        922      924       +2     

Impacted Files	Coverage Δ
dpdata/deepmd/hdf5.py	80.76% <33.33%> (-1.90%)	⬇️

---

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 d9e400f...2b1f81b. Read the comment docs.