Erased two lines of code which should not be present in models.py in previous commit.
Tally computation uses weight for voter groups. When computing the vote of each voter, the vote is multiplied as many times as the value of the weight.
If computation is done in the backgroud, by calling tasks.election_compute_tally.delay(), the weights seem to be ignored. No idea why for the moment!
Thus, computation is not done in the backgroud now (by not using .delay in calling the function).