added lagged value functionality for retrieving values further down in memory; Converted position always to lower case strings to avoid problems with caps sensitivity names; added examples for use in examples.js

src/measurements/Measurement.js

@@ -67,6 +67,23 @@ class Measurement {
     if (this.values.length === 0) return null;
     return this.values[this.values.length - 1];
   }
+
+  getLaggedValue(lag){
+    if(this.values.length <= lag) return null;
+    return this.values[this.values.length - lag];
+  }
+
+  getLaggedSample(lag){
+    if (lag < 0) throw new Error('lag must be >= 0');
+      const index = this.values.length - 1 - lag;
+    if (index < 0) return null;
+
+    return {
+      value: this.values[index],
+      timestamp: this.timestamps[index],
+      unit: this.unit,
+    };
+  }
   
   getAverage() {
     if (this.values.length === 0) return null;