Add additional tests for Matrix, IntegerMatrix, and DoubleMatrix

src/SparseTransform/DataStructures/DoubleMatrix.cs

@@ -18,7 +18,7 @@ namespace DataStructures
                 {
                     for (int j = 0; j < ColumnCount; j++)
                     {
-                        if(this[i, j] != 0)
+                        if (this[i, j] != 0)
                         {
                             nonZeros++;
                         }
@@ -43,7 +43,7 @@ namespace DataStructures
         public String ToMatrixMarketString()
         {
             String MMString;
-            String type = "double";
+            String type = "real";
             String symmetry;
             if (Symmetric)
             {
@@ -61,9 +61,10 @@ namespace DataStructures
             for (int i = 0; i < RowCount; i++)
             {
                 for (int j = 0; j < ColumnCount; j++)
-                {
-                    MMString += $"{i} {j} {this[i, j]}\n";
-                }
+                    if (this[i, j] != 0)
+                    {
+                        MMString += $"{i} {j} {this[i, j]}\n";
+                    }
             }
             return MMString;
         }