Rework external events

Rework the external Event code and permits users to input generic events that are forwarded to the scheduler without having to code everything in Batsim.

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.

merged