We have a visitor now for generating
 - the XML
 - the Module's dot graph
 - the textual dump of the VariableNetwork
 - the dot graph of the VariableNetwork

Changed an exception into an assert, since the function is only internally used and the exception is never thrown. Clafiried the documentation about this.

When optimising the connections (i.e. merging networks), the trigger receivers for triggered networks are now removed from the trigger network. This was previously leading to crashes.

renamed VariableNetwork::addTriggerReceiver() into VariableNetwork::addNodeToTrigger() for better clarity

Made some exception descriptions more elaborate. To achieve this, the dump() functions have been extended to allow writing to an arbitrary ostream.

Adapation to the unified versions of ControlSystemAdapter and DeviceAccess.

Status: Library compiles, tests do not yet compile.

- 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.

- 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

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.

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.