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[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e948661) 82.88% compared to head (dbe6699) 82.86%.

Files	Patch %	Lines
dpdata/plugins/qe.py	60.00%	2 Missing :warning:
dpdata/qe/traj.py	85.71%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #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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