Add getKey() method

app/static/app/js/classes/Units.js

@@ -177,6 +177,7 @@ class UnitSystem{
     volumeUnit(cbmeters, opts = {}){ throw new Error("Not implemented"); }
     
     getName(){ throw new Error("Not implemented"); }
+    getKey(){ throw new Error("Not implemented"); }
     
     area(sqmeters, opts = {}){
         sqmeters = parseFloat(sqmeters);
@@ -237,6 +238,10 @@ class MetricSystem extends UnitSystem{
         return _("Metric");
     }
 
+    getKey(){
+        return "metric";
+    }
+
     lengthUnit(meters, opts = {}){
         if (opts.fixedUnit) return units.meters;
 
@@ -263,6 +268,10 @@ class ImperialSystem extends UnitSystem{
         return _("Imperial");
     }
 
+    getKey(){
+        return "imperial";
+    }
+
     feet(){
         return units.feet;
     }
@@ -314,6 +323,10 @@ class ImperialUSSystem extends ImperialSystem{
         return _("Imperial (US)");
     }
 
+    getKey(){
+        return "imperialUS";
+    }
+
     feet(){
         return units.feet_us;
     }