…an run with higher priority over local

As it is, nodeGroup messages are not checked until all local and regular Charm queue (prio Q) messages are
checked, which cause issues when the applicaiton is using nodeGroup messages in the hope that *some* PE will attend to it quickly. The change makes getNextMessage check nodeGroup queue every 2^nodeGrpFreq iterations with
high priority in addition to its usual check after exhasuting local queues (except task Q).
This commit has not been tested at all. But pusing it to allow others to help me test/fix it.