#418 Document intended PLC runtime structure.

Adding a document to discuss and document PLC program runtime structure.

Compared with previous code generators, this design is closer to the PLC capabilities. As a result, the gap between CIF and the PLC code becomes larger. I tried to limit the impact by less freedom in the CIF input and/or more pre-processing.

While this is a merge request and I think the document is useful to keep around, a second goal is to discuss this proposal, and perhaps find alternatives, or other improvements in the CIF to PLC conversion process.

Addresses #418 (closed)