#782 PLCgen: Improve documentation in the transition code

Readable by commit, the last 2 are big as they update resp add a test. A unit test evolves with the commits to see what gets changed in the output.

Adds

• A header with involved automata in an event.
• List of edges being considered for the various steps in the process.
• Single line comments what the next code is doing or what conclusions can be drawn.
• channel.cif test with senders, receivers, syncers and monitors.

Closes #782 (closed)