Add more documentation and checks to the models

Models/ThemeConfig/Json/LayerConfigJson.ts

@@ -84,7 +84,9 @@ export interface LayerConfigJson {
 
     /**
      * This tag rendering should either be 'yes' or 'no'. If 'no' is returned, then the feature will be hidden from view.
-     * This is useful to hide certain features from view. Important: hiding features does not work dynamically, but is only calculated when the data is first renders.
+     * This is useful to hide certain features from view. 
+     * 
+     * Important: hiding features does not work dynamically, but is only calculated when the data is first renders.
      * This implies that it is not possible to hide a feature after a tagging change
      *
      * The default value is 'yes'

Models/ThemeConfig/Json/TagRenderingConfigJson.ts

@@ -79,6 +79,10 @@ export interface TagRenderingConfigJson {
         /**
          * If this condition is met, then the text under `then` will be shown.
          * If no value matches, and the user selects this mapping as an option, then these tags will be uploaded to OSM.
+         * 
+         * For example: {'if': 'diet:vegetarion=yes', 'then':'A vegetarian option is offered here'}
+         * 
+         * This can be an substituting-tag as well, e.g. {'if': 'addr:street:={_calculated_nearby_streetname}', 'then': '{_calculated_nearby_streetname}'}
          */
         if: AndOrTagConfigJson | string,
         /**

Models/ThemeConfig/LayerConfig.ts

@@ -104,6 +104,10 @@ export default class LayerConfig {
                 throw context + "Use 'geoJson' instead of 'geoJsonSource'";
             }
 
+            if (json.source["geojson"] !== undefined) {
+                throw context + "Use 'geoJson' instead of 'geojson' (the J is a capital letter)";
+            }
+
             this.source = new SourceConfig(
                 {
                     osmTags: osmTags,
@@ -133,6 +137,14 @@ export default class LayerConfig {
                 const key = kv.substring(0, index);
                 const code = kv.substring(index + 1);
 
+                try{
+                    
+                new Function("feat", "return " + code + ";");
+                }catch(e){
+                 throw `Invalid function definition: code ${code} is invalid:${e} (at ${context})`   
+                }
+
+
                 this.calculatedTags.push([key, code]);
             }
         }

Models/ThemeConfig/LayoutConfig.ts

@@ -53,10 +53,10 @@ export default class LayoutConfig {
     public readonly cacheTimeout?: number;
     public readonly overpassUrl: string;
     public readonly overpassTimeout: number;
-    private readonly _official: boolean;
+    public readonly official: boolean;
 
     constructor(json: LayoutConfigJson, official = true, context?: string) {
-        this._official = official;
+        this.official = official;
         this.id = json.id;
         context = (context ?? "") + "." + this.id;
         this.maintainer = json.maintainer;
@@ -221,7 +221,7 @@ export default class LayoutConfig {
     }
 
     public CustomCodeSnippets(): string[] {
-        if (this._official) {
+        if (this.official) {
             return [];
         }
         const msg = "<br/><b>This layout uses <span class='alert'>custom javascript</span>, loaded for the wide internet. The code is printed below, please report suspicious code on the issue tracker of MapComplete:</b><br/>"