title: writing a matrix into a single txt file with mpi
body: <p>I have a huge matrix that I divided it into some sub matrices and I make some computation on it. After those computations I have to write that matrix into a single file for post processing. Is it possible to write results into a single text file and how can I do it?
  For example we have a nx<em>ny matrix that is divided in y direction (each processes has a nx</em>rank matrix) and we want to write the nx*ny matrix into a single text file.</p>