Fix: Change index names in MMReader

* Old terminology was inconsistent with rest of the program and most of
  the literature on matrices
* x corresponded to i and y to j which is an uncommon mapping of the
  axes
* This does not change how the graph nodes are created from the indices

src/SparseTransform/Convert/MatrixMarketReader.cs

@@ -127,9 +127,9 @@ namespace SparseTransform.Convert
                         {
                             throw new ArgumentException("Syntax Error in MatrixMarket file: Data line did not contain three elements.");
                         }
-                        int xCoordinate = Int32.Parse(values[0]);
-                        int yCoordinate = Int32.Parse(values[1]);
-                        processor(xCoordinate, yCoordinate, Double.Parse(values[2]));
+                        int i = Int32.Parse(values[0]);
+                        int j = Int32.Parse(values[1]);
+                        processor(i, j, Double.Parse(values[2]));
                     }
                 }
             }