fix reading of vlen strings

[ManuelHu]

Geant4 writes strings as variable length datatype, and they end up in LH5 files via the remage-to-lh5 conversion.

h5py reads them as object arrays, and attaches a vlen metadata property, so return a string datatype in this case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.38%. Comparing base (0104086) to head (b44409d).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   78.30%   78.38%   +0.07%     
==========================================
  Files          46       46              
  Lines        3384     3400      +16     
==========================================
+ Hits         2650     2665      +15     
- Misses        734      735       +1     

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