Prevent confusion if someone uses selectCount() (which returns int) on a

grouped query. They should be using select(Funcs.count(Exps.literal("*")))
(which returns List<Integer>) in such cases.

src/main/java/com/samskivert/depot/Query.java

@@ -359,6 +359,7 @@ public class Query<T extends PersistentRecord>
      */
     public int selectCount ()
     {
+        checkState(_groupBy == null, "You surely mean to select(Funcs.count(Exps.literal(\"*\"))).");
         _fromOverride = new FromOverride(_pclass);
         return _repo.load(CountRecord.class, _cache, getClauseArray()).count;
     }