[Dune] g++ support

[Dedner, Andreas]

Hi.
Hope you are all well...

I wanted to check, what the required g++ version now is?
In dune-common/README.md it says >=7 but for a few days now std::optinal is used in dune-common and that seems to fail with 7.3
which is the compiler our CI in Stuttgart is using with the Debian 10
image.
Best
Andreas
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