avoid run the TractFlow merge if the results exist
avoid run the TractFlow merge if the results exist

thekingofkings/urban-flow-analysis,thekingofkings/chicago-crime,thekingofkings/chicago-crime,thekingofkings/urban-flow-analysis,thekingofkings/chicago-crime,thekingofkings/chicago-crime,thekingofkings/urban-flow-analysis,thekingofkings/urban-flow-analysis,thekingofkings/urban-flow-analysis,thekingofkings/chicago-crime,thekingofkings/chicago-crime