Bugfix: calculate row and column from linear index

The calculation of row and column index back from a linear index
failed because of a mix-up in the functions. The bugfix has been
tested with the following script:

 DoubleMatrix operant = DoubleMatrix.zeros(200, 200);
 for (int rowtest = 0; rowtest < operant.rows; rowtest++) {
    for (int coltest = 0; coltest < operant.columns; coltest++) {
       int linear = operant.index(rowtest, coltest);
       if (rowtest != indexRows(operant, linear)) {
          System.err.println("Fail row check for "+rowtest+"x"+coltest +" (linear: "+linear+") resulted in: "+indexColumns(operant, linear));
       }
       if (coltest != indexColumns(operant, linear)) {
          System.err.println("Fail column check for "+rowtest+"x"+coltest +" (linear: "+linear+") resulted in: "+indexColumns(operant, linear));
       }
    }
 }

I the test, no error message has been displayed