Feature: Trap SIGTERM and close gracefully

[wdconinc]

Is your feature request related to a problem? Please describe.
Sometimes interrupting a long simulation run is necessary. Currently this fails to close the root file correctly.

Describe the solution you'd like
Ctrl-C should end the run properly as if the last event was reached.

Describe alternatives you've considered
More periodic ROOT writing may work too.

Additional context
I just prefer how Kassiopeia is handling this :-)