code: public Matrix subtract(Matrix B, ExecutorService threadPool)
      {
          Matrix toReturn = getThisSideMatrix(B);
          toReturn.mutableSubtract(1.0, B, threadPool);
          return toReturn;
      }
docs: Creates a new Matrix that stores the result of <i>A-B</i>
  @param B the matrix to subtract from <i>this</i>.
  @param threadPool the source of threads to do computation in parallel
  @return a new matrix equal to <i>A-B</i>