runInEpoch* do not take a universal reference (they take a r-val ref)

[lifflander]

What Needs to be Done?

Due to this, you can pass an existing std::function to them. Template them and move to the header file. Something like this would work:

template <typename Callable>
void runInEpochCollective(Callable&& fn) {
  //...
}

[lifflander]

Also, write tests for these to avoid problems like this coming up

[cz4rs]

Due to this, you can pass an existing std::function to them. Template them and move to the header file. Something like this would work:

Changes implemented, typo in the description? (it was not possible to pass std::function before and it is possible now, after templating the functions)