useVspace datareducers need multipop support

[alhom]

At least here the correct population needs to be called:

analysator/pyVlsv/vlsvreader.py

Line 1032 in 86b5e6c

velocity_cell_data = self.read_velocity_cells(singlecellid)

[alhom]

analysator/pyVlsv/vlsvreader.py

Line 3035 in 9322d69

return []

So this either should return an empty dict for the calling functions to not fail: return {}, or raise a proper error.