Consider making data sources and generator functions consistent in their default return types

[BjoernLudwig]

At the moment we have two options to set a signal source:

1. Either you set up a DataStreamMET4FOF with a data source consisting of the three tuple quantities, target, and time, or
2. you set up a generator function returning the two tuple quantities, and time.

Should we simplify dealing with signals by unifying the default interfaces?