#559 Add PLCgen expression generator.

Theoretically this should convert CIF expressions to PLC equivalents. Given a CIF expression it outputs a PLC expression with additional PLC statements if necessary.

@jverbakel I added an interface for getting PLC equivalents of CIF data at org.eclipse.escet.cif.plcgen.conversion.expressions.CifDataProvider, you may want to have a look at it.

I haven't run this code yet it needs tests, but I thought it to be a good idea to show what I have so far. I'll write and add tests (with fixes most likely).

Commits are meant to be read in sequence although at the end I fixed a few things.
The generator started as a mostly commented-out copy of the cif2plc generator, and then parts are replaced. Hopefully if you read it side-by-side you can use that old information.

Fixes #559 (closed) Addresses #397 (closed) #418 (closed)

[[Removed the "draft" state]]