Add flag for datareducer preference in read_variable

[alhom]

https://github.com/fmihpc/analysator/blob/1d8b270fcddb87fbf97a763ef7db3d7b516a880c/pyVlsv/vlsvreader.py#L1680

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.