PLCgen: allow to customise name of variables connected to the I/O address

The PLC code generator generates names (tags) for Inputs and Outputs (I/O) using an I/O table file. The third column of that file contains the absolute names of the CIF variables connected to the I/O address. These names are set and cannot be customized. There is a need to customize these names (tags).

A proposed solution for this problem is to add a fourth column in the I/O table file where a customized name can be entered. For example: %I20.0,BOOL,HardwareMapping.SimpleModel_HW.I_aan,MyCustomInputName

Addresses #679