LA: add Vector.AsArray to directly return internal arrays if the schema is matching

src/Numerics/LinearAlgebra/Matrix.cs

@@ -1376,7 +1376,6 @@ namespace MathNet.Numerics.LinearAlgebra
         /// Otherwise returns null. Changes to the returned array and the matrix will affect each other.
         /// Use ToArray instead if you always need an independent array.
         /// </summary>
-        /// <returns>A multidimensional containing the values of this matrix.</returns>
         public T[,] AsArray()
         {
             return Storage.AsArray();

src/Numerics/LinearAlgebra/Storage/DenseVectorStorage.cs

@@ -329,6 +329,20 @@ namespace MathNet.Numerics.LinearAlgebra.Storage
             }
         }
 
+        // EXTRACT
+
+        public override T[] ToArray()
+        {
+            var ret = new T[Data.Length];
+            Array.Copy(Data, 0, ret, 0, Data.Length);
+            return ret;
+        }
+
+        public override T[] AsArray()
+        {
+            return Data;
+        }
+
         // ENUMERATION
 
         public override IEnumerable<T> Enumerate()

src/Numerics/LinearAlgebra/Storage/SparseVectorStorage.cs

@@ -611,6 +611,18 @@ namespace MathNet.Numerics.LinearAlgebra.Storage
             }
         }
 
+        // EXTRACT
+
+        public override T[] ToArray()
+        {
+            var ret = new T[Length];
+            for (int i = 0; i < ValueCount; i++)
+            {
+                ret[Indices[i]] = Values[i];
+            }
+            return ret;
+        }
+
         // ENUMERATION
 
         public override IEnumerable<T> Enumerate()

src/Numerics/LinearAlgebra/Storage/VectorStorage.cs

@@ -374,6 +374,23 @@ namespace MathNet.Numerics.LinearAlgebra.Storage
             }
         }
 
+        // EXTRACT
+
+        public virtual T[] ToArray()
+        {
+            var ret = new T[Length];
+            for (int i = 0; i < ret.Length; i++)
+            {
+                ret[i] = At(i);
+            }
+            return ret;
+        }
+
+        public virtual T[] AsArray()
+        {
+            return null;
+        }
+
         // ENUMERATION
 
         public virtual IEnumerable<T> Enumerate()

src/Numerics/LinearAlgebra/Vector.cs

@@ -238,15 +238,23 @@ namespace MathNet.Numerics.LinearAlgebra
 
         /// <summary>
         /// Returns the data contained in the vector as an array.
+        /// The returned array will be independent from this vector.
+        /// A new memory block will be allocated for the array.
         /// </summary>
-        /// <returns>
-        /// The vector's data as an array.
-        /// </returns>
+        /// <returns>The vector's data as an array.</returns>
         public T[] ToArray()
         {
-            var result = new DenseVectorStorage<T>(Count);
-            Storage.CopyToUnchecked(result, ExistingData.AssumeZeros);
-            return result.Data;
+            return Storage.ToArray();
+        }
+
+        /// <summary>
+        /// Returns the internal array of this vector if, and only if, this vector is stored by such an array internally.
+        /// Otherwise returns null. Changes to the returned array and the vector will affect each other.
+        /// Use ToArray instead if you always need an independent array.
+        /// </summary>
+        public T[] AsArray()
+        {
+            return Storage.AsArray();
         }
 
         /// <summary>