#815 PLCgen: add more detailed comments to generated PLC code

Patch is readable by commit.

Inserts comment lines above the code that output the affected CIF variable when:

• Initializes a state variable.
• Updates the remaining time of a continuous variable.
• Changes a state variable in an update.
• Changes a state variable by reading PLC input.
• Writes a state variable to PLC output.

In addition, it inserts information about the model structure before the program. Currently for each complex component (for as far as useful):

• Variables of the automaton.
• Uncontrollable events it participates in.
• Controllable events it participates in.
• The PLC edge selection variable of the automaton component if it has one.

I briefly looked into the edge numbers, and concluded they are not trivial, and need constants or enumerations at least. Similarly, we have magic location numbers or enumerations. These look like a good idea to add to the model view too, but probably better at a different time.

Closes #815 (closed) Addresses #679