Add aggregate API

Weather/Service/Models/ReadingAggregate.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using ChrisKaczor.HomeMonitor.Weather.Models;
+using JetBrains.Annotations;
+
+namespace ChrisKaczor.HomeMonitor.Weather.Service.Models
+{
+    [PublicAPI]
+    public class ReadingAggregate
+    {
+        public decimal Min { get; set; }
+
+        public decimal Max { get; set; }
+
+        public decimal Average { get; set; }
+
+        public ReadingAggregate(IEnumerable<WeatherReading> readings, Func<WeatherReading, decimal> selector, int decimalPlaces)
+        {
+            Min = readings.Min(selector);
+            Max = readings.Max(selector);
+            Average = readings.Average(selector).Truncate(decimalPlaces);
+        }
+    }
+}