Change particle postprocessor writer from raw pointers to unique pointers

[MFraters]

Based on comments in #4979. This brings the writer more in line with the writer in visualizer.cc

[bangerth]

This will not work. You start a new thread, and that thread has a reference to the std::unique_ptr object file_contents. But by the time the background thread dereferences it, (i) the string the pointer points to may already have been de-allocated, and (ii) the std::unique_ptr itself may no longer be a valid object. You have to capture everything you need here by copy, and you need to take over ownership of the pointer. In visualization.cc, this looks as follows:

                    = std::thread([ my_filename = std::move(filename),
                                    my_temporary_output_location = temporary_output_location,
                                    my_file_contents = std::move(file_contents)]()
                  {
                    writer (my_filename, my_temporary_output_location, *my_file_contents);
                  });

[bangerth]

I will note that this was already wrong for the filename and temporary_output_location variables. I think we only got lucky by the fact that the thread immediately calls writer, which copies these objects because these arguments are passed by value. This copy must have happened quickly enough that the main function did not already exit.

[MFraters]

hmm, oke. I did see that it was done slightly different in visualization.cc, but I wanted to keep the code as little as possible while introducing the unique_ptr. I will now just make it the same as there (which I probably should just have done from the start)

[MFraters]

fixed it

[bangerth]

Yes, great, thank you!