Implement steps 2 and 4 of escet plcgen

Steps 2 and 4 deal with CIF types and variables.

Step 3 is skipped for now because constants cannot be shown for PLC targets that don't support constant definitions, until plcgen has support for using constants in some context.

From #397 (closed):

2. Handle type declarations or simple types. Needs

• finding type declaration in CIF model,
• notion of types in the transformation,
• target-specific mapping of CIF data types to PLC data types, for the basic types (bool, int, real)
• notion of types in the PLC code,
• type equality and ordering

4. Extract discrete variables from the CIF model, organize them by automaton, and generate variable tables and initialization of the variables for each supported PLC system. Needs

• processing CIF model and storing found information by model (there should be previous art),
• converting CIF discrete variables to PLC while keeping the connection between both (CIF reading or writing a variable must eventually generate access to its PLC equivalent),
• constructing and writing tables of variables for each support PLC system (global variables?), and
• constructing an initialization step in the generated PLC.

The "type equality and ordering" item in step 2 doesn't seem very needed at this point, and may be skipped as well.