Warn if column count is wrong for this nharm value

src/prep.py

@@ -46,6 +46,10 @@ def main(argv = None):
         print "  nilmtool -u %s create %s %s" % (e.dest.url, e.dest.path, rec)
         raise SystemExit(1)
 
+    if f.dest.layout_count != args.nharm * 2:
+        print "error: need", args.nharm*2, "columns in destination stream"
+        raise SystemExit(1)
+
     # Check arguments
     if args.column is None or args.column < 1:
         parser.error("need a column number >= 1")