XX is outside 4-byte uint range

Created by: darafferty

When trying to do an averager step with freqstep=6, I get the error "6 is outside 4-byte uint range". I traced the message to line 385 of https://github.com/lofar-astron/DP3/blob/master/Common/StringUtil.cc. Changing the < to > fixed it, but I'm not sure this is really the correct fix.