Fix #394 for #389 does not work with get_data_set

[lpetitTUD]

A minimal example:

import time
import numpy as np
import qcodes
from qcodes.tests.data_mocks import DataSet2D
from qcodes.tests.instrument_mocks import DummyInstrument

dataset = DataSet2D()
A = DummyInstrument(server_name='test')
sweepvalues=A.dac2[0:100:.05]

#%%
#%Loop in background
print(qcodes.get_bg())

loop=qcodes.Loop(sweepvalues, progress_interval=.2, delay=.001).each(A.dac1)
data=loop.get_data_set() # data_manager=True)
print('created dataset...')
#data=loop.run(background=True)
data=loop.run(background=True, data_manager=True)
#data=loop.run()
print('sync %s' % data.sync())
print('fraction %.3f' % data.fraction_complete())
data.complete() # should wait on data to be ready

for ii in range(20):    
    data.sync()
    print('sync %s' % data.sync())
    print('fraction %.3f' % data.fraction_complete())
    time.sleep(0.2)