THE April Criminal Sessions will be officially opened today in Georgetown with a ceremonial parade, the inspection of a guard of honour by one of the High Court Judges, who will also take the salute during a march past. Two hundred and thirty-four cases are listed for trial by two judges.   Between them they are expected to deal with 75 murders, six attempted murders, 21 manslaughters, two narcotics, 64 carnal knowledge and 23 rapes.