Introduced the DeviceModule class which provides an interface to obtain VariableNetworkNodes for device registers

- performed some code cleaning and refactoring
- store all nodes in a VariableNetwork in a single list, to allow deciding later if a node is feeding or consuming

This commit is an intermediate commit and has known bugs. Almost all tests are failing and the demoApp is not running. Debugging will be done in a next step.

introduced an operator>> and operator<< syntax to connect network nodes. to realise this, the VariableNetworkNode class has been changed into a pimpl pattern.

Allow to make connections directly based on VariableNetworkNodes. This is a first step, more changes have to be made in this direction.

Use the threaded version of the FanOut if a pushing feeder is distributed to multiple consumers, so all consumers receive updates immediately.

- fixed a bug in the FanOut which did not wait for the trigger (since ProcessScalar::receive() does not wait)
- fixed a bug in Application::feedDeviceRegisterToControlSystem() not handling the last arugment (trigger) properly
- improved the output produced with VariableNetwork::dump()

added also the consumer-type variables to the XML file

Add some first code to produce an XML file describing the application variables published to the control system. This is not yet doing anything useful for now...

start the synchronisation thread of the FanOut to "convert" a poll-type feeder into a pushing feeder by putting in an external trigger.

Intermediate commit before the weekend: added options to add an external trigger to a network to handle poll and push properly. Currently it does not work properly.

renamed publishDeviceReadRegister() and publishDeviceWriteRegister() into feedDeviceRegisterToControlSystem() and consumeDeviceRegisterFromControlSystem()

renamed publish() and connectToDevice() into more specific feedToControlSystem()/consumeFromControlSyste() and feedToDevice()/consumeFromDevice()

improved structure of network creation and added a FanOut to distribute values to multiple consumers

Introduced the VariableNetwork describing the connections of variables/accessors before things get actually instantiated. This is needed to generate an XML file containing the published variables based on that information.

This is a first mock implementation with very limited functionality. The idea is based on discussions during the OPC-UA control system adapter kickoff meeting and subsequent personal discussions.