Added getEpubVersionString to OpfItem

[lasselehtinen]

The current getEpubVersion returns only int, which is unsufficient for our use when we would need to the exact ePub version number (2.0.1, 3.0, 3.3 and so on). Instead of changing the current functionality, I added a new getEpubVersionString method to OPF parser which returns the full version as string.

[ewilan-riviere]

Excellent idea, thanks for this PR!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.34%. Comparing base (18997c1) to head (f7c29fa).
Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #110      +/-   ##
============================================
+ Coverage     90.33%   90.34%   +0.01%     
- Complexity      833      834       +1     
============================================
  Files            32       32              
  Lines          2577     2580       +3     
============================================
+ Hits           2328     2331       +3     
  Misses          249      249              

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