update ToString to adopt Scalar value

Author: Oceania2018

src/NumSharp.Core/Casting/NdArray.ToString.cs

@@ -11,8 +11,27 @@ public partial class NDArray
         public override string ToString()
         {
             string output = "";
-
-            if (this.ndim == 2)
+            if (this.ndim == 0)
+            {
+                switch (dtype.Name)
+                {
+                    case "Int16":
+                        output = Data<short>()[0].ToString();
+                        break;
+                    case "Int32":
+                        output = Data<int>()[0].ToString();
+                        break;
+                    case "Double":
+                        output = Data<double>()[0].ToString();
+                        break;
+                    case "String":
+                        output = Data<string>()[0].ToString();
+                        break;
+                    default:
+                        throw new NotImplementedException("NDArray ToString()");
+                }
+            }
+            else if (this.ndim == 2)
             {
                 if(dtype == typeof(int))
                 {

src/NumSharp.Core/NDStorage.cs

@@ -178,7 +178,14 @@ public int DTypeSize
         {
             get
             {
-                return Marshal.SizeOf(_DType);
+                if(_DType == typeof(string))
+                {
+                    return 0;
+                }
+                else
+                {
+                    return Marshal.SizeOf(_DType);
+                }
             }
         }
         /// <summary>