Logging clean-up

[alhirzel]

Would a pull request be accepted to clean up logging and unify under Memento.jl or Logging.jl? Right now there is a mix of ModiaLog and println usage, which can make debugging models less than straightforward. I would propose mapping the existing logging strategy onto one of those frameworks. This would also ease the integration of Modia into other packages (e.g. GUI packages).