Extend the constructor of DatastreamMet4FoF and MetrologicalDatastreamMet4FoF

[BjoernLudwigPTB]

At the moment DatastreamMet4FoF provides a default generator function which is used, when set_generator_function() is called without specifying a function. Since we agreed today, that there actually is no reasonalbe generic signal for a general Datastream, we might as well exchange this default behaviour and specifically ask the user to specify a signal source on initialization with the constructor signature

def __init__(self, data_source: Any = None, generator_function: Callable = None):

and check in the constructor for at least one of these two being specified. Ín the MetrologicalDatastreamMet4FoF we could then either expect the generator_function and data_source to return values and uncertainties or introduce an additional uncertainty_function().