Feature/std chrono parser

What this MR does / why does DuMux need it:

Adds a helper header for working with std::chrono, and for now adds a function Dumux::Chrono::toSeconds that takes a string and returns an instance of std::chrono::duration (representing seconds). Dumux::Chrono::toDuration(duration, string) allows parsing into a different duration (e.g. milliseconds, etc)...

Draft because it has to wait for !3648 (merged)

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