There seems to be a bug with an occurence of self.value_unc instead of self._value_unc in metrological_streams

We should investigate if [lines 80-82](https://github.com/Met4FoF/agentMET4FOF/blob/0a2ada913e2bb6ede3351436743d350f6dde1506/agentMET4FOF/streams/metrological_base_streams.py#L81-L83) should be corrected. Looks to me as if some rename refactorization went wrong.