Soft kill ptask

Implements the soft kill of ptasks. (Probably) fixes #108 (closed) and #37 (closed)

Pipeline

Branch name.

• Descriptive and short
• Use hyphens to separate words

Branch content.

• Only dedicated to the problem.
• Based on Batsim's official master branch.
• Straightforward. Just a sequence of commits. Does not contain merge commits.
• Test results are not worse than before. How to run Batsim tests?