Multiprocess performance on Windows

Nobody has claimed this yet.

HPCToolkit reader is hanging on Windows, seems to be a difference in child processes between Linux and WIndows. On Windows, the error is as follows:

```
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.

This probably means that you are not using fork to start your
child processes and you have forgotten to use the proper idiom
in the main module:

if __name__ == '__main__':
freeze_support()
...

The "freeze_support()" line can be omitted if the program
is not going to be frozen to produce an executable.
```

https://www.pythonforthelab.com/blog/differences-between-multiprocessing-windows-and-linux/

Start at the HPCToolkit reader's multiprocessing entry point and reproduce the hang on Windows, comparing its process-start behavior with Linux. Done means the reader completes without the bootstrap RuntimeError or hanging when multiprocessing is used on Windows.

Written by the indexing model from the issue text.