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.

added missing cxx and linker flags from dependencies, removed some remaining traces of direct DOOCS dependency variables

also export the LIBRARY_DIRS variable to the calling CMakeLists.txt (needed to publish it to further downstream dependencies)