Don't highlight overlaps in single models

Don't show a dotted line around overlapping entities in the single
model view.

Refs pombase/pombase-gocam#63

Author: kimrutherford

gocam_view/templates/gocam_view/index.html

@@ -608,7 +608,7 @@
 },
 'border-style': function(el) {
   let style = 'solid';
-  if (isConnectingNode(el)) {
+  if (isMergedModel && isConnectingNode(el)) {
     style = 'dashed';
   }
   return style;