implement the "testable mode" in which the application can be paused and resumed by the test routine

Added a TestDecoratorRegisterAccessor which is optionally put around accessors to facilitate test of applications. This TestDecoratorRegisterAccessor does not yet do anything, functionality will be added in the next commits.

fixed replacing Array and ScalarAccessors, which was broken after moving to the NDRegisterAccessorBridge-based accessors

fixed poll-type inputs using no longer non-blocking read after the switch to the NDRegisterAccessorBridge-based accessors

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

Changed the FeedingFanOut so that it does not extract the meta-data (name, shape etc.) from the first slave but expects it in the constructor. This is necessary since the meta-data is already given by the application.

fixed variable networks with external trigger not being put into the same TriggerFanOut (which is required since they should be in the same TransferGroup)

- add all networks triggered from the same node to the same TriggerFanOut
- put all feeding nodes in a TriggerFanOut into the same TransferGroup

- added some class documentation

- some more code cleaning

fixed blocking read transfer for the constant accessor: should only return on first read and then block forever

Forward declaration is taken from DeviceAccess header file, which solves problems if classes are moved to another namespace.