Add qprocess job launching

[MBartkowiakSTFC]

Description of work
We switch from QThread to multiprocessing.Process for starting new jobs. This way the jobs will not be competing for the Python GIL.

Fixes
A State pattern has been implemented for jobs.
A context menu lets the user pause, restart and terminate jobs from the GUI.
Each job is a process and uses pipes to communicate with the main process.

To test
Run a few longer jobs at the same time. See if they all use 100% of a CPU core. Try pausing and resuming them.

[ChiCheng45]

A few issues I've found from my testing so far.

• For an non-technical user they probably won't know the exact differences between kill and terminate. Might be just better to have just one of them.
• Closing the MDANSE main window leaves jobs running. This would be that the user would need to kill the job manually in the task manager.
• Job kill/pause does seem to work correctly with trajectory conversion jobs.

• Percentage complete can go over 100% in some cases.

[MBartkowiakSTFC]

Thanks for finding these problems!
Regarding the jobs that keep running after the GUI has been closed: do they finish normally, and produce correct results?

[ChiCheng45]

Thanks for finding these problems! Regarding the jobs that keep running after the GUI has been closed: do they finish normally, and produce correct results?

I just tried it with a trajectory conversion and I closed the main window before it finished. After waiting some time it produced an MDT file which loads successfully with a new MDANSE.

So yea it appears to finish successfully.

[MBartkowiakSTFC]

That's good, really. Thanks for checking!

I will fix the other problems in this PR, and for the processes outliving the GUI we will have to make a separate issue.

[ChiCheng45]

Everything seems to work from my testing.

The only thing is that when I run a trajectory conversion, the MDT file are created before its completes. When I terminate the job the files remain. I've created #342 for this issue.