[WIP] Signal handling - SIGTERM will stop calculation and force file saving

[bigfooted]

Proposed Changes

Special signal handling for SIGTERM. When SIGTERM is sent to the solver, we catch it and simply exit the for-loop for the number of iterations. Files will then automatically be saved.

Related Work

Related to Github discussion #2094

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[bigfooted]

Here is a simple proof-of-concept for the fluid iteration where we catch SIGTERM. If you run it and hit ctrl-c, we catch the signal and we set stopcalc to true. This will exit the for-loop and also forces a saving of the files.
I think implementing the signal handler like this is the best, because we let the solver handle all the cleanup.

[bigfooted]

We probably also have to do something like this at other places where we have external iterations.

[pcarruscag]

This should be at the driver level, not inside iteration classes otherwise it won't work for multizone.
How are signals handled by MPI? Can this result in a race condition?

[bigfooted]

OK, so I am moving the signal handling to COutput::ConvergenceMonitoring as discussed in the dev-meeting.

[bigfooted]

@pcarruscag This works for me. Do you think this is OK or we need to do it different/more, maybe a config keyword to activate/deactivate, or other options?

[pcarruscag]

I have not had time to test it for single/multi zone, steady/unsteady, and primal/adjoint.