PLCgen: add more detailed comments to generated PLC code

Add more comments to the generated PLCgen, for input reading, state initialization/updating, and output writing, similar to how already done for transition code (#782 (closed)).

Currently planned:

• For input reading: at least per variable that is written.
• For state initialization: at least per variable that is written.
• For timer updates: at least per variable that is written.
• For output writing: at least per variable that is read.

Addresses #679