Add flag for datareducer preference in read_variable

[alhom]

analysator/pyVlsv/vlsvreader.py

Line 1680 in 1d8b270

def read_variable(self, name, cellids=-1,operator="pass"):

Sometimes one might want to force using datareducer instead of a stored value (e.g. some known issue in saved values). Adding a flag to handle that should be simple.