Vector: Added array constructor - keeps a reference, does not make a copy

Signed-off-by: Marcus Cuda <marcus@cuda.net>

src/Numerics/LinearAlgebra/Double/DenseVector.cs

@@ -103,6 +103,17 @@ namespace MathNet.Numerics.LinearAlgebra.Double
             }
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DenseVector"/> class for an array.
+        /// </summary>
+        /// <param name="array">The array to create this vector from.</param>
+        /// <remarks>The vector does not copy the array, but keeps a reference to it. Any 
+        /// changes to the vector will also change the array.</remarks>
+        public DenseVector(double[] array) : base(array.Length)
+        {
+            Data = array;
+        }
+
         /// <summary>
         ///  Gets the vector's data.
         /// </summary>

src/UnitTests/LinearAlgebraTests/Double/DenseVectorTests.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using MbUnit.Framework;
 
 namespace MathNet.Numerics.UnitTests.LinearAlgebraTests.Double
 {
@@ -21,5 +22,20 @@ namespace MathNet.Numerics.UnitTests.LinearAlgebraTests.Double
 
             return vector;
         }
+
+        [Test]
+        [MultipleAsserts]
+        public void CanCreateVectorFromArray()
+        {
+            var data = new[] { 1.0, 2.0, 3.0, 4.0 };
+            var vector = new DenseVector(data);
+            Assert.AreSame(data, vector.Data);
+            for( var i = 0; i < data.Length; i++)
+            {
+                Assert.AreEqual(data[i], vector[i]);
+            }
+            vector[0] = 100.0;
+            Assert.AreEqual(100.0, data[0]);
+        }
     }
 }