Add a conversion operator to std::string for podio::version::Version

[veprbl]

BEGINRELEASENOTES

• Added an operator for conversion to std::string for podio::version::Version

ENDRELEASENOTES

[tmadlener]

Can we make this an explicit operator? I am not the biggest fan of implicit conversions, and in this case the difference for users would be acceptable IMHO.

const auto versionStr = std::string(version);

rather then

const std::string versionStr = version;

Unless you have a specific use case this would fix where the explicit cast would be too cumbersome?

[veprbl]

I also prefer the explicit conversion, but it's not so popular in C++ world.