moved VariableNetworkNode member to AccessorBase class, so the network node information can be accessed also when having only the pointer to the base class

Added description field to the variables. It is currently ignored and will be added to the XML file in a next step.

Adapation to the unified versions of ControlSystemAdapter and DeviceAccess.

Status: Library compiles, tests do not yet compile.

- extended the trigger tests
- removed some debug output

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.

Remove the restriction which prevented connecting nodes of the same direction. This was an artificial restriction, since later checks make sure that the network has exactly one feeder.

added also the consumer-type variables to the XML file

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 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.