Statistics: DescriptiveStatistics.Median now obsolete

Median always requires a at least a second pass, for exact evaluation of
unsorted data even to keep all data in memory (depending on
implementation). There is thus no advantage in including it in
DescriptiveStatistics, dropping it will instead make it more useful
and perform much better (both runtime and memory usage).

src/Numerics/Statistics/DescriptiveStatistics.cs

@@ -157,6 +157,7 @@ namespace MathNet.Numerics.Statistics
         /// Gets the sample median.
         /// </summary>
         /// <value>The sample median.</value>
+        [Obsolete("Dropped in future versions for performance reasons. Please use Statistics.Median instead.")]
         public double Median { get; private set; }
 
         /// <summary>