Rebased the ScalarAccessor and ArrayAccessor classes onto the mtca4u::NDRegisterAccessorBridge class.
Known problems present: code compiles but tests do not run

Fixed a bug that a pointer to a temporary VariableNetworkNode object was sometimes stored (as a trigger receiver or source). Instead now an object with the same data pointer inside is stored.

Read all application inputs once before starting the module threads, so the initial values are correctly present.

For this purpose, additional changes were necessary:
- Added support for constant values as feeders and consumers (just ignore the writes)
- Connect all unconnected application accessors to constants after defining the connections and before creating the implementations
- Renamed the user-overridden function "initialise()" into "defineConnections()", since more actions are now necessary during initialise().
- The connections are now made (i.e. the implementations are created) during the initialisation phase

allow specifying the array length for direct connections between device and control system (as the type is already defined in this case)

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.