Include LatLongDetector on DetectorsDescriptor<T> (#3384)

Closes #3383

Author: russcam

src/Nest/XPack/MachineLearning/Job/Detectors/Detector.cs

@@ -285,5 +285,8 @@ public DetectorsDescriptor<T> TimeOfDay(Func<TimeDetectorDescriptor<T>, ITimeDet
 
 		public DetectorsDescriptor<T> TimeOfWeek(Func<TimeDetectorDescriptor<T>, ITimeDetector> selector = null) =>
 			Assign(a => a.AddIfNotNull(selector.InvokeOrDefault(new TimeDetectorDescriptor<T>(TimeFunction.TimeOfWeek))));
+
+		public DetectorsDescriptor<T> LatLong(Func<LatLongDetectorDescriptor<T>, IGeographicDetector> selector = null) =>
+			Assign(a => a.AddIfNotNull(selector.InvokeOrDefault(new LatLongDetectorDescriptor<T>())));
 	}
 }

src/Nest/XPack/MachineLearning/Job/Detectors/GeographicDetector.cs

@@ -44,8 +44,11 @@ public class LatLongDetectorDescriptor<T> : DetectorDescriptorBase<LatLongDetect
 		Field IPartitionFieldNameDetector.PartitionFieldName { get; set; }
 		Field IFieldNameDetector.FieldName { get; set; }
 
+		[Obsolete("Use parameterless constructor")]
 		public LatLongDetectorDescriptor(string function) : base(function) {}
 
+		public LatLongDetectorDescriptor() : base(GeographicFunction.LatLong.GetStringValue()) {}
+
 		public LatLongDetectorDescriptor<T> FieldName(Field fieldName) => Assign(a => a.FieldName = fieldName);
 
 		public LatLongDetectorDescriptor<T> FieldName(Expression<Func<T, object>> objectPath) => Assign(a => a.FieldName = objectPath);