boost::range::join performance issue

[mathbagu]

• Do you want to request a feature or report a bug?
  Bug

• What is the current behavior?
  When we iterate over all the buses from the Network bus view, the performance decrease after several voltage levels. Doing a classic for-loop offer acceptable performance.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

// This for-loop is really slow
for (const Bus& bus : network.getBusView().getBuses()) {
    std::cout << bus.getId() << std::endl;
}

// This for-loop is fast
for (const VoltageLevel& vl : network.getVoltageLevels()) {
    for (const Bus& bus : vl.getBusView().getBuses()) {
        std::cout << bus.getId() << std::endl;
    }
}

• What is the expected behavior?
  Iterate over the buses of the busView efficiently.

• What is the motivation / use case for changing the behavior?
  Increase the performance.

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)

[mathbagu]

Maybe the boost::range::join is not adapted to what we need, and we should implement a flatten iterator:
https://stackoverflow.com/questions/44508979/boost-range-adaptor-flattened
https://stackoverflow.com/questions/3623082/flattening-iterator

[mathbagu]

boostorg/range#118