Use C++ interface of VILLASnode

We should switch to the C++ interface of VILLASnode in the VILLASinterface class to be compatible with the latest and coming versions of VILLASnode. This entails significant changes in the VILLASinterface class.

First and foremost, we have use the "parse" functions of VILLASnode to set the config params of each node. Until now, we access and set these params manually. This is no longer possible because now they are "hidden" behind protected or private members of the VILLASnode Node class.

I suggest to move the configuration params for the VILLASinterface from the model.props to a separate json file (and specify the json file as config property in the model.props) so that the format of the config is similar to that of VILLASnode. We can read the json file in each process and hand the contents as json_t (using libjansson) to the VILLASinterface class.

This approach has one disadvantage: We would need to configure the signals (signal names, types, units) in the json file as well because they have to be known for the parsing process. This collides with the current approach of the VILLASmessage class that is used to derive and define a specific message format for an agent behavior and to facilitate the actual transfer of data between VILLASnode and DistAIX. We need to rethink this approach in order to avoid duplication of code and signal definitions.

If have started the work here 390dab5d.

Comments are welcome @felix.wege