Enable loading of virials for 'qe/cp/traj' if prefix.str file is present

[rashidrafeek]

Currently the virials are not read when reading a QE cp.x MD. But the stresses are printed in the prefix.str by default. This PR adds support to load virials if this file is present.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.86%. Comparing base (e948661) to head (dbe6699).
⚠️ Report is 251 commits behind head on master.

Files with missing lines	Patch %	Lines
dpdata/plugins/qe.py	60.00%	2 Missing ⚠️
dpdata/qe/traj.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #583      +/-   ##
==========================================
- Coverage   82.88%   82.86%   -0.03%     
==========================================
  Files          74       74              
  Lines        6608     6617       +9     
==========================================
+ Hits         5477     5483       +6     
- Misses       1131     1134       +3     

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[njzjz]

Could you add a test for virial?

dpdata/tests/test_qe_cp_traj.py

Line 9 in e948661

class TestCPTRAJProps: