Signal handling

added kill signal handler to shut down the solver cleanly. This is useful when serializing solutions of large problems that cannot be solved to optimality in reasonable time

assigned to @tluchterha

enabled an automatic merge when the pipeline for 068d9650 succeeds

aborted the automatic merge because source branch was updated

added 2 commits

• 4a301f26 - 1 commit from branch master
• fdee1315 - Merge branch 'master' into signal_handling

Compare with previous version

mentioned in commit 9001699e

merged