Refactor position handling in MeasurementContainer to use position values instead of names

src/measurements/MeasurementContainer.js

@@ -88,11 +88,11 @@ class MeasurementContainer {
     return this;
   }
 
-  position(positionName) {
+  position(positionValue) {
     if (!this._currentVariant) {
       throw new Error('Variant must be specified before position');
     }
-    this._currentPosition = positionName;
+    this._currentPosition = positionValue;
     return this;
   }
 
@@ -243,10 +243,12 @@ class MeasurementContainer {
     const savedPosition = this._currentPosition;
     
     // Get upstream and downstream measurements
-    this._currentPosition = 'upstream';
+    const positions = this.getPositions();
+
+    this._currentPosition = Math.min(...positions);
     const upstream = this.get();
     
-    this._currentPosition = 'downstream';
+    this._currentPosition = Math.max(...positions);
     const downstream = this.get();
     
     this._currentPosition = savedPosition;