#400 Implement first step in escet plcgen

Closes #400 (closed)

Initial setup for escet-plcgen, mainly infra-structure files and minimal code files. Basically the program reads a CIF file and ignores it, and writes an empty project for each target. Due to limitations in cif2plc though it's not completely empty, for example it outputs timers and some other files. I think this is fine for now.

I borrowed heavily from cif2plc (mostly by importing) for producing PLC output, except for the ABB target as that doesn't exist. For this target I also looked at the plcgen0 version, but I don't like it. It's optimized for copy/paste into the ABB development environment. For example it omits everything that the environment provides, such as the header and footers of functions and tables. It's not a complete PLC program that you can read or review.