necessary changes for the latest DeviceAccess update (readAsync() return type no longer a reference, and other related changes)

update for interface change in DeviceAccess: TransferElement::write() has now the version number as an argument

Fixed several bugs affecting poll-type variables being read by a poll-type application variable and distributed to push-type variables via a ConsumingFanOut.

- use this return type in testable mode to detect when a queue overflow happened (testable mode counter should not increase in that case)

use the new readLatest(), which has been added to the latest DeviceAccess version, to implement the naturally expected behaviour when using a push-type variable with a poll-type accessor

- added some class documentation