Add support for cryptografic PRNG in netstandard1.6

Use a different way to initialize the PRNG creation for the .NET
framework as described on this SO answer:
https://stackoverflow.com/a/38644970

Add System.Security.Cryptography.Algorithms NuGet on netstandard1.6.

src/Numerics/Numerics-2017.csproj

@@ -22,10 +22,9 @@
         <Reference Include="System.Runtime.Serialization" />
     </ItemGroup>
     <ItemGroup Condition="'$(TargetFramework)' == 'netstandard1.6'">
-        <PackageReference Include="System.Runtime">
-          <Version>4.3.0</Version>
-        </PackageReference>
+        <PackageReference Include="System.Runtime" Version="4.3.0" />
         <PackageReference Include="System.Runtime.Serialization.Xml" Version="4.3.0" />
         <PackageReference Include="System.Runtime.Serialization.Formatters" Version="4.3.0" />
+        <PackageReference Include="System.Security.Cryptography.Algorithms" Version="4.3.0" />
     </ItemGroup>
-</Project>
+</Project>

src/Numerics/Random/CryptoRandomSource.cs

@@ -27,7 +27,7 @@
 // OTHER DEALINGS IN THE SOFTWARE.
 // </copyright>
 
-#if !PORTABLE && !NETSTANDARD1_6
+#if !PORTABLE
 
 using System;
 using System.Collections.Generic;
@@ -51,7 +51,7 @@ namespace MathNet.Numerics.Random
         /// <see cref="Control.ThreadSafeRandomNumberGenerators"/> to set whether the instance is thread safe.</remarks>
         public CryptoRandomSource()
         {
-            _crypto = new RNGCryptoServiceProvider();
+            _crypto = RandomNumberGenerator.Create();
         }
 
         /// <summary>
@@ -71,7 +71,7 @@ namespace MathNet.Numerics.Random
         /// <param name="threadSafe">if set to <c>true</c> , the class is thread safe.</param>
         public CryptoRandomSource(bool threadSafe) : base(threadSafe)
         {
-            _crypto = new RNGCryptoServiceProvider();
+            _crypto = RandomNumberGenerator.Create();
         }
 
         /// <summary>
@@ -135,12 +135,12 @@ namespace MathNet.Numerics.Random
             var bytes = new byte[values.Length*4];
 
 #if !NET35
-            using (var rnd = new RNGCryptoServiceProvider())
+            using (var rnd = RandomNumberGenerator.Create())
             {
                 rnd.GetBytes(bytes);
             }
 #else
-            var rnd = new RNGCryptoServiceProvider();
+            var rnd = RandomNumberGenerator.Create();
             rnd.GetBytes(bytes);
 #endif
 
@@ -168,7 +168,7 @@ namespace MathNet.Numerics.Random
         /// <remarks>Supports being called in parallel from multiple threads.</remarks>
         public static IEnumerable<double> DoubleSequence()
         {
-            var rnd = new RNGCryptoServiceProvider();
+            var rnd = RandomNumberGenerator.Create();
             var buffer = new byte[1024*4];
 
             while (true)

src/Numerics/Random/RandomSeed.cs

@@ -6,10 +6,10 @@ namespace MathNet.Numerics.Random
     {
         static readonly object Lock = new object();
 
-#if PORTABLE || NETSTANDARD1_6
+#if PORTABLE
         static readonly System.Random MasterRng = new System.Random();
 #else
-        static readonly System.Security.Cryptography.RandomNumberGenerator MasterRng = new System.Security.Cryptography.RNGCryptoServiceProvider();
+        static readonly System.Security.Cryptography.RandomNumberGenerator MasterRng = System.Security.Cryptography.RandomNumberGenerator.Create();
 #endif
 
         /// <summary>
@@ -41,7 +41,7 @@ namespace MathNet.Numerics.Random
         {
             lock (Lock)
             {
-#if PORTABLE || NETSTANDARD1_6
+#if PORTABLE
                 return MasterRng.NextFullRangeInt32() ^ Environment.TickCount ^ System.Guid.NewGuid().GetHashCode();
 #else
                 var bytes = new byte[4];