Control: choose default linear algebra provider by env var

Setting the variable MathNetNumericsLAProvider=MKL
will now make Control default to the MKL provider.

If non is set or setting it fails, it will fall back
to the managed provider as before.

src/Numerics/Control.cs

@@ -28,6 +28,8 @@
 // OTHER DEALINGS IN THE SOFTWARE.
 // </copyright>
 
+using MathNet.Numerics.Algorithms.LinearAlgebra.Mkl;
+
 namespace MathNet.Numerics
 {
     using System;
@@ -66,7 +68,26 @@ namespace MathNet.Numerics
             CheckDistributionParameters = true;
             ThreadSafeRandomNumberGenerators = true;
             DisableParallelization = false;
-            LinearAlgebraProvider = new ManagedLinearAlgebraProvider();
+
+            try
+            {
+                const string name = "MathNetNumericsLAProvider";
+                var value = Environment.GetEnvironmentVariable(name);
+                switch (value != null ? value.ToUpper() : string.Empty)
+                {
+                    case "MKL":
+                        LinearAlgebraProvider = new MklLinearAlgebraProvider();
+                        break;
+                    default:
+                        LinearAlgebraProvider = new ManagedLinearAlgebraProvider();
+                        break;
+                }
+            }
+            catch
+            {
+                // We don't care about any failures here at all
+                LinearAlgebraProvider = new ManagedLinearAlgebraProvider();
+            }
         }
 
         /// <summary>