make RoutingModel::IsModelClosed() as public access to RoutingModel::closed_

[N40]

What language and solver does this apply to?
C++

Describe the problem you are trying to solve.
Check whether or not CloseModel... was called and avoid running into LOG(WARNING) << "Model already closed"

Describe the solution you'd like

bool RoutingModel::ModelIsClosed() const
{
  return closed_;
}

Describe alternatives you've considered
QuiteCloseModelWithParameters() could also / instead be made public