Change const_iterator to iterator to remove const_cast in BCM_RodDecoder.

http://goetz.web.cern.ch/goetz/mutable_2019-06-10.html#Cnv_BCM

nightly/master/2019-06-10T2153:InnerDetector/InDetEventCnv/BCM_RawDataByteStreamCnv/src/BCM_RodDecoder.cxx +382

   380	  }
   381	  if (collExists) {
   382	    coll = const_cast<BCM_RDO_Collection*>(&**cont_it);
   383	  } else {  // if collection does not exist create it
   384	    coll = new BCM_RDO_Collection(chan);