Calling yield and delay outside of tasks

[Laxilef]

Hello.
Please add yield and delay methods to SchedulerClass
This will allow you to make changes to other classes that are used inside tasks.
I tested this on one of my projects and it doesn't affect how groups work.

Little changes are needed:

void SchedulerClass::begin() {
  current = current->next = first;
  while (1) {
    updateCurrentTask();

    if (current->shouldRun())
      cont_run(&current->context, task_tramponline);

    updateRunGroups();

    ::yield();

    current = current->next;
  }
}

void SchedulerClass::delay(unsigned long ms) { current->delay(ms); }
void SchedulerClass::yield() { current->yield(); }

[nrwiersma]

This should be in the pull request section.