Bug fix: shift variable set but not used in sink density loop
This MR fixes the error 'runner_do_nonsym_pair_sinks_naive_density': runner_doiact_functions_sinks.h:193:10: error: variable 'shift' set but not used [-Werror=unused-but-set-variable]
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I added the shift term to the computation of the sink position, which is how it is done for the BHs.