saddle-points: make MatrixCoordinate implement Comparable

The saddle-points tests make it seem like you can use any kind of set
for your output. However, if you use TreeSet then the tests will fail
because MatrixCoordinate doesn't implement Comparable.

MatrixCoordinate does now implement Comparable to preent this failure.

exercises/saddle-points/.meta/src/reference/java/MatrixCoordinate.java

-class MatrixCoordinate {
+class MatrixCoordinate implements Comparable<MatrixCoordinate> {
     private final int row;
     private final int col;
 
         result = 31 * result + col;
         return result;
     }
+
+    @Override
+    public int compareTo(MatrixCoordinate o) {
+        int rowComparison = Integer.compare(row, o.row);
+        return (rowComparison == 0) ? Integer.compare(col, o.col) : rowComparison;
+    }
 }

exercises/saddle-points/src/main/java/MatrixCoordinate.java

-class MatrixCoordinate {
+class MatrixCoordinate implements Comparable<MatrixCoordinate> {
     private final int row;
     private final int col;
 
         result = 31 * result + col;
         return result;
     }
+
+    @Override
+    public int compareTo(MatrixCoordinate o) {
+        int rowComparison = Integer.compare(row, o.row);
+        return (rowComparison == 0) ? Integer.compare(col, o.col) : rowComparison;
+    }
 }