Threading: CommonParallel now supports empty loops. Closes gh-19.

CommonParallel did break on empty loops with an argument exception instead
of just doing nothing. It now exits early if there's nothing to do in
For and Invoke, no longer throwing an exception. Also, it now special cases
single-element iterations and inlines their execution (trivial loop
unrolling), skipping all the threading/task overhead.

src/Numerics/Threading/CommonParallel.cs

@@ -72,6 +72,20 @@ namespace MathNet.Numerics.Threading
         /// <exception cref="AggregateException">At least one invocation of the body threw an exception.</exception>
         public static void For(int fromInclusive, int toExclusive, Action<int> body, bool parallel)
         {
+            // Special case: no action
+            if (fromInclusive >= toExclusive)
+            {
+                return;
+            }
+
+            // Special case: single action, inline
+            if (fromInclusive == (toExclusive - 1))
+            {
+                body(fromInclusive);
+                return;
+            }
+
+            // Common case
             if (parallel)
             {
 #if SILVERLIGHT
@@ -353,6 +367,20 @@ namespace MathNet.Numerics.Threading
         /// <exception cref="AggregateException">An action threw an exception.</exception>
         public static void Invoke(params Action[] actions)
         {
+            // Special case: no action
+            if (actions.Length == 0)
+            {
+                return;
+            }
+
+            // Special case: single action, inline
+            if (actions.Length == 1)
+            {
+                actions[0]();
+                return;
+            }
+
+            // Common case
             var maxThreads = Control.DisableParallelization ? 1 : Control.NumberOfParallelWorkerThreads;
 #if SILVERLIGHT
             Parallel.Invoke(actions);