tweak trains and trainlines displayed zoom (#412)

Co-authored-by: Pierre-Yves Rollo <dev@pyrollo.com>

public/js/components/LayerSelector.js

@@ -7,9 +7,11 @@ function onchange(e){
   m.route.set("/map/:layerId/:zoom/:lon/:lat", params);
 }
 
-
 export default {
   view: function(){
+    // Display layer selector only if there is choice
+    if (LayerManager.layers.length <= 1)
+      return null;
 
     const layers = LayerManager.layers.map(layer => m(
       "option",

public/js/map/overlays/TrainOverlay.js

@@ -71,7 +71,7 @@ export default L.LayerGroup.extend({
 
 
   getMaxDisplayedZoom: function(){
-    return 10;
+    return 7;
   },
 
   createMarker: function(train){

public/js/map/overlays/TrainlineOverlay.js

@@ -58,6 +58,10 @@ export default AbstractGeoJsonOverlay.extend({
       }
     });
   },
+
+  getMaxDisplayedZoom: function(){
+    return 4;
+  },
   
   createGeoJson: function(objects){
     var self = this;